The aim here is to isolate the edges ourselves on another layer. By blending the layers we an control how much of affect the sharpness has, we can also change the masking easily at any time to avoid sharpening places that don't benefit from it.
1) Duplicate your layer, name new layer high pass
2) With the new layer selected do Filters >> Edge Detect >> Edge, lets try the default settings, so hit ok.
3) We can see on this abstract image what consists of an edge, lets invert the colors. Color >> Invert.
4) The inverted image would be really useful to us if all that white did not exist and was transparent, Color >> Colour to Alpha, pick the from color as white (FFFFFF), it shold be the default. Hit ok and the high pass will be overlayed, but the color will look crazy.
5) Make the High Pass layer grayscale, colors >> desaturate. This should look like someone took a dark pencil and drew an outline around the defining parts of your subject.
6) Drop the opacity to 50.0 and change layer mode to Overlay, which will blend it more subtley. Try click eye icon next to the layer name to turn it on and off and see what it is doing.
7) Congratulations, GIMP does not have adjustment layers but you have just made a makeshift sharpness adjustment layer. If you want the affect to be stronger move the opacity towards 100, if you want a more subtle affect move the opacity towards 0.
8) Part of the benefit of having this on a separate layer is we can apply a mask. If you accidentally ended up sharpening something you don't want you can apply a layer mask and mask it out. You can also do creative sharpening here by using shades of gray in the layer mask. White = Sharpened, Black = Original, grays are somewhere in the middle.
Here is two images, top is untouched bottom has the affect as described here, notice the contrast is not affected (good), the noise is not enhanced (good) but the image looks sharper (very good). USM is so 1997 
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Creative Options:
We have glimpsed edge detect. By default you have 5 different edge filters, and each can be tweaked, don't be afraid to try the others. My personal favorite is Neon, especially for working with portraits and bring out detail in hair. We have also glimpsed layer blending modes, but only used normal and overlay (there are 21 in total). We have a lot of control here. Later we will talk about dodging and burning. But we have those as layer blend modes also, so theoretically we could make a dodge/burn adjustment layer quite easily also, stay tuned for more on that. We also can use edge detected grayscale images as layer masks, and channels (a quick mask is channel, so that means selections too).
Even Easier?:
Oh its easy, for me this is easier than USM as the noise is almost never an issue, and if it becomes and issue, its easy to mask out. So while I take a few more steps total time is less than it would be playing around with USM sliders. You can get scripts from registry.gimp.org that do all these steps for you. Unfortunately most of them merge the final high pass with the original, so you loose the ability to go back and make changes if you use them.
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