Excel spreadsheet on my PDA. It is always with me, and I just have the sheet set up so that each cell is 15 minutes. It is very simple, but at least it doesn't take up a lot of time.

I seem to recall a very polished online service that let you break down your time working, and what you were working on, via a simple interface in a pop-up window. I just googled for online timesheets, and apparently there are a million such services. I guess I'd jump to one of them if I was doing more contract work.