I've research this extensively when my company wanted a time tracking solution. From memory, here were some of the best ones I found:
Harvest - Web based with a really slick interface. There is a companion iPhone application that proved to be a killer feature.
Freshbooks - Online accounting software that also has a nice iPhone application.
Toggl - Seems fairly nice. Note nearly as many features as the ones previously mentioned (at least not when I looking into it a few months back)
Tick - Another really nice interface. They also have an iPhone application.
Pen and Paper - My favorite because its easiest. Lets you go back and enter your time when its convenient for you.