I've checked out this question, but it refers to something a little different.
I would like an OS X app that is like a stopwatch with multiple timers. So if I have five different project and I start working on project "A" I can start that clock and then stop it and start the project "B" clock, etc.
The reason I want this is so that I can get a more accurate sense of how I spend my time throughout the day. My ambition is to use this knowledge to increase my productivity.
An iPhone app would also be an option.