There are quite a few programs out there for the PC and the Mac; if you're a PC user, you can search for "time tracking" on
Download.com where you'll find programs like this one:
Time Tracker - Reviews and free Time Tracker downloads at Download.com
You can do the same search for mac software at
MacUpdate. Obviously, an Excel spreadsheet would do the trick, too (what these other programs buy you are reporting and invoicing capabilities, easy tracking by projects, "stopwatch" capabilities for tracking time, etc. that you wouldn't get with a spreadsheet).
Vasily