Couldnt' find a thread on this, found on the gopro forums.
Free, just need a s60 or winmobile 6 phone
For those of you who do trackdays, there is a free GPS based software called RaceCrono that you can use for lap timing and other data logging:
Afterwards you can export all kinds of data to an AVI file and combine it with your GoPro video by using the RaceChrono2AVI software:
From the FAQ:
RaceChrono works on phones with Symbian S60 2nd edition feature pack 1 and above, any S60 3rd edition or 5th edition. Also Windows Mobile 5 and 6 are supported, non-touchscreen and touchscreen devices.
Accuracy of the lap times is fully dependent on quality of your GPS signal. If GPS signal is weak, RaceChrono can be inaccurate and will probably even miss your traps (will not record a lap time). GPS signal quality is affected by your GPS receiver quality and its placement. Lap timing with a normal 1 Hz GPS and RaceChrono achieves +-0.25 seconds accuracy or better, and with 5 Hz GPS the accuracy will be better than +-0.15 seconds.