GIMP ships with at least 9 ways of selecting things, maybe more. 7 of these are intended for selection only, the other 2 have other uses but also can be used to select things. The main 7 ways share some common features such as 4 modes of selection. These 4 modes are Replace/Add (Shift)/Subtract (Ctrl)/Intersect (Shift + Ctrl), explained below. They also all allow you to feather the selection, which is another way of saying you want the edges of the selection to be soft and not hard, it blurs the edge of the selection by an amount you specify, useful later on when you use layers.
All of these support the Quick Mask also, so don't worry if you selection is not perfect, you don't have to start again, you can make very subtle adjustments to selections if you need to using the mode or the masks.
Using the Rectangle tool as an example of the 4 select modes (you will be familiar with it from tutorial 1 on cropping).
1) The default mode is Replace. This means every time you draw a rectangle you are making a brand new selection and ignoring the old one. Probably what you expect the tool to do.
2) If you select the Add mode from the tool options (hint: its a picture of 2 red squares) or if you just hold down the shift key on the keyboard, every time you draw a rectangle it will be added to the current selection. So you can draw anything from 1 to infinity boxes on screen at the same time. Great if you first selection didn't get quite enough, no need to start again just add some more.
3) The Subtract mode (1 red, 1 white box, or hold down CTRL) works in a similar way except it removes a piece of the selection. So for instance you could select a square of 500x500, then then subtract a square of 400x400 from the center of that selection and would be left with a selection in the shape of a hollow box that is 100 pixels wide. Great also if you selected too much, no need to start again, just subtract some selection.
4) Intersect I've never used on a real image. If you draw a box, then select intersect mode (white box, red box, white box) then draw a box that overlaps an existing selection. You will be left with a selection made of only the part of the 2 selections that overlapped.
So lets look at the 7 main tools and the options they give, the first few are simple but limited with complex selections, the last few are very smart for complex selections but not great for simple selections. So while the Sicssor tool and forground select may look cool, the simple tools are just as valid.
Part 1: The Simple Tools
Rectangle Select (R)
* Simple way to draw a square or rectangle, great for cropping to other image sizes or picking out square/rectangle shapes.
* Supports all 4 selection modes
* Allows you to feather the selection
* Allows you to round the corners of the rectangle
* Can expand from centre. This means that when you click and drag the mouse the box expands from the centre outwards in all directions. Normally it starts at the point you click and expands in the direction you drag the mouse.
* Can fix aspect ratio and/or size as seen in cropping tutorial
* Can highlight selection and show guides as seen in crop tutorial
Ellipse Select (E)
* Simple way to draw circle/oval shapes
* Supports all 4 selection modes
* Anti aliasing (leave this on)
* Feather edges
* Expand from center
* Fixed aspect and/or size, highlights and guides.
Free Select (F)
* Freehand drawing, you can hold the mouse button and draw in one fluid movement, or you can click point to point, and any combination of both. The selection is not made until you join back to the point you started selecting from. Its fast and accurate enough, you can always tweak the selection later if its not perfect.
* Supports all 4 selection modes
* Supports anti aliasing for removing jagged edges on curves
* Supports feathering edges
Fuzzy Select Tool (U)
* The first extra smart tool, select areas based on colours.
* Anti aliasing (leave this on)
* Feather edges
* Can select transparent areas also
* Sample merged (means base decision on all layers not just the active one)
* Threshold slider and mode: This is how you tell the tool how much of a difference in color is enough. Default is 15. If you set this to 0 likely not much will be selected unless its a solid block of the same color, it would really need a crazy amount of contrast to work. If you set this to 100, likely too much would be selected. So if you clicked on a yellow shirt on a pale person it might also select all their skin. The default 15 is pretty good starting point. If it selects a little too little you can either bump it up, or if you want to be sensible about it, press shift and click on the little bit it missed out to add it to the selection. Remember its Fuzzy its not supposed to nail everything in one click, that's a strength not a weakness 
Select by Color (Shift+O)
* Supports, 4 modes, Anti aliasing, feather, transparent areas, sample merged
* Almost identical to the Fuzzy Select Tool with one major difference. If you had a picture of a group of people and you clicked on someones red shirt with the fuzzy tool, you'd get that persons shirt. With this tool you get everything that red. Someone else has red eyes, you got that, someone else has red shoes, you also get that. So you might get too much "stuff", not to worry you can select red with this, then switch to one of the other tools like Free Select, and subtract the extra bits from your selection. You can keep this tool and add/subtract colors instead of stuff also. Want to get rid of all red and green, click something red, hold shift click something green, both are selected.
Part 2: The Complex Tools
Scissor Tool (I)
Now this boy is smart, smart, smart, this will save you a bunch of time. It has the 4 modes also, but they work like paths now. Don't worry I'll explain paths later, the basic headline is you can draw an approximate shape around your target using multiple points, the tool will try and figure out what it should be selecting based on your points and gravitate around that. Its surprisingly accurate, but if it misses don't worry, you can just move that point, or add another point (shift key), or remove a point that might be complicating its decisions (CTRL key). DON'T click inside the selection until you are happy with it as it will effectively become a fixed selection that you can't edit anymore. When you are happy, just click inside it to convert it from its path+intelligence form to its selection form. Attachment 1 shows an unfinished selection with the scissor tool. Some of these points where too far and had to be moved closer to the item. The top edge will have another point or two added (ctrl+click) to help it realize that is an edge. It appeared to be fooled by the shadow, the bottom edge had no such issue. A perfect selection of a relatively complex item with about 20 seconds work 
Foreground Select ()
I'll be the first to admit this looks hugely powerful, but I did not get on with it at all until I start writing the paragraph, I'm used to manually editing the selection mask, but I can see this is the same with a little bit of intelligence added, after using it for 5 minutes I can see it will be my new best friend.
In use this is like the evil lovechild of the free select tool, and a sort of quick mask mode. What you do is draw freehand style a loose outline around your target. Everything outside your target will turn blue. Then you can refine the foreground selection (what you want selected) by painting on any part you missed. If you missed any part of your target paint over it. The paint will be black. When you release the mouse it will take the hint you just gave it and apply some smarts to the selection and hopefully be closer to what you want. You can also give it hints of the background (what you don't want selected) by holding ctrl and painting the background with white paint, again release the mouse, give it a second to try interpret what you mean. It could take a few passes to get it perfect. When you have what you want, press the enter key and your selection is made.
Attachment 2 shows an unfinished selection with the foreground select tool, I need to paint over the silver bit at the top of the lighter and let it figure out I want that bit to finish it. When I'm happy I press enter and the screen goes back to normal colours with the lighter selected.
Part 3: The old school manual tools
Paths converted to Selections
The other 2 ways I know about, where used before Scissor Tool and Foreground Select. Scissors works like Paths in that you can draw any shape you like and add/remove points until you like it. Paths can be converted to selections, but they don't have the smartness to grab an edge of an object, so it could be a very labor intensive process to do this is the past. Paths are still very useful for other things, we will go into later, but surpassed by the scissor tool for selections in the majority of cases.
Painting on the Quick Mask
Foreground select is similar to using a quick mask. Whats a quick mask? Well lets look at one, take the rectangle tool, select a rectangle and press Shift+Q the screen will turn red (not blue like the foreground tool) with the selection transparent. The idea is the same, paint what you want included in the selection white, and what you don't want selected black (it will show as red or transparent). The difference again, is its manual, no smartness to help you get close to that edge. The benefit? Well you can paint things many shades of gray also. In fact when you feather a selection you are applying a blur to the selection mask, so that the edge between black(excluded) and white(included) becomes a bit gray, so you don't get a hard edge to your selection. More on masks in a later topic, press Shift+Q to go back to normal mode.
Attachment 1: Scissor Tool before final adjustments
Attachment 2: Foreground Tool before final adjustments
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. It'd be nice to have a sticky w/ all your writeups.
