Any Brobot v2
It'd be the "sweet million" key in this picture.
The function would be to end my ridiculous student debt.Show Image(https://farm8.staticflickr.com/7393/12940433654_3ea3d3d149_o.jpg)
Transpose current glyph and next one; as someone who types in a fashion that produces a lot of transpositions, this would be efficient.In Mac OS X, almost all text boxes support using Ctrl-T to transpose characters. It’s also pretty straightforward to change this or other keybindings if you want.
Terminal key. No matter what program you're in, it would drop down a command prompt/terminal ala QuakeI actually have this as meta+~. It's pretty nice to have sometimes. I personally use a program called "Yakuake", but there are other programs too. Or if you use something like awesome/xmonad/other crazy configurable wms (I usually do, but have been using kde recently to try it out again) you can set it up with any terminal emulator inside your wm.
As a programmer it would be nice to have a key that would insert comment blocks / characters correctly for the giving programming language in question regardless of the text editor and OS. This is course is a pipe dream, but it would still be something nice.+1