Time tracking software

Does anyone know of an open source application that will:
Track which applications are open and active (being used, not just running in the background), when they were opened and for how long?
Cross platform would be nice, although this would be used at the moment on XP SP2.
Generate reports? Email would also be nice.
This would be for billing purposes, so the punch in-punch out timer type apps would probably not be the best way to go.
I guess in a pinch, freeware would be acceptable as well.

Thanx in advanceā€¦

pixelmass,

Here are a few hits from sourceforge. I have never used any of these, so I cannot recommend them.

For applications using flexlm

Another possibility?

Best of Luck!