Huge unstable stacks of books. Remarkably, they won't need whatever book you pick out for them. Go to Amazon or BUY.COM and get a gift certificate. Nothing says 'geek' like an expensive PDA on your belt. The Palm Pilot is the industry leader and an excellent choice.
Caffeine is always good.
You can't go wrong with an espresso maker from Best of Italy . Of course, I have a cheap espresso maker and it produces caffeine just fine. The Leatherman Wave is as nice as they come and a serious bargain at around USD 60. Geeks like things from Edmund Scientific . When in doubt go with a telescope or a GPS system, but anything from this site will be appreciated by your geek.