I'm almost done developing Lap Timing software. If you've got a buddy and a laptop, you can use it to keep track of your lap times. Actually, the only thing left for me to do is complete the average speed conversion for MPH/KPH and vice-versa.
The software has an unlimited lap memory and recall, calculates average lap time and average lap speed (if you enter the track length), and keeps track of the fastest lap.
My next step will be to port it to Windows CE (Pocket PC), and to the Palm OS as well. These options would be a bit more realistic in terms of what someone might actually use at the track, although I have seen my share of laptops at the track!
If you're interested, you can check it out on my personal website