Clicking a button to turn on your computer isn't particularly difficult, but if you're looking for a more personalised way to turn your PC on, Code and Life shows off how to turn your computer on with a knock.
The build here costs about $US10 and uses an ATtiny45, piezo sensor, PS/2 connector and a few other minor items. The end result is a switch that will turn your computer on (or off) with a knock. It's a simple setup that shouldn't take long to make. You can find the full how to over on Code and Life.
Turning PC On with a Knock Using ATtiny45 and a Piezoelectric Sensor [Code and Life via Hack a Day]