This is my DIY guide to making your own cheap and reliable variable length OC-E2 or OC-E3 cable.
Firstly some background: I am not electrically trained beyond taking a few classes in electronics for my school certificate ~10 years ago. This involves nothing more complex than cutting wires and poking a soldering iron at some wires. I make no claims to have done this as well as it could have been done nor as neatly as it could have been done, however... most people who are interested in this I imagine have a level of skill equitable to mine so I think it's a good example of what you can do with nothing more than basic knowledge and skills.
Why would you do it? Allowing you to use your sole flash as an off-camera flash with a 100% reliable connection - including ETTL, if that's your cup of tea. It's not wireless like Poverty Wizards but it'll work every time and with enough slack I don't find the cable inconveniencing, so I know which one I'd rather work with.
With newer cameras like my 1D III and other models (40D onwards? I don't recall precisely) you can control the flash using the camera's controls. Not a big deal if the flash is right there on your hotshoe. However... using a very long OC-E3 you can then put your master flash in your lighting setup while still using it to trigger the other flashes, rather than wasting its light output on top of your camera (assuming you're not using some on-axis light). Given that it's cheaper and offers more control than a ST-E2... you get everything a 580EX can do. Also opens up some possibilities of where you can be and still trigger your flashes using Canon's own triggering system.
Being able to change the length as required is also handy... no point in lugging 10m of cable around for when you only want 2 or 3. Also, Cat5 cable is cheap. Very cheap. You break it on a job, lose it, have your dog eat it, whatever... no big deal to replace.
Items used: Soldering iron (mine cost $10 or so), solder, 1m Cat5 cable which can be had for <$3 in most places... and something to cut with. I used a Swiss Army Knife, anything will do.
Skill required: Essentially none. To be able to do simple soldering... I picked up a soldering iron a few times in high school - 10 years ago now. Not once since until this mod, so if I can do it anyone can.
I'm linking to the photos for now, will come back and embed them in case the source links change.
Onwards to the good bits!
First, take your OC-E3 and take out the screws in the underside of both ends.