Another alternative people should consider: if you have an Android-based phone, you can pick up a high-res external GPS unit fairly cheap (e.g. this one on Amazon which is what I use):
http://www.amazon.com/BT-Q818XT-Qstarz-10Hz-Bluetooth-Receiver/dp/B003YCZINA, and use it with one of a couple of android apps depending on your focus. For road course and after-analysis, I prefer
aLapRecorder HD, but if you want to see your splits displayed in a big font *while* you're lapping, you might prefer
Trackmaster (downside is, it doesn't record OBD-II, while aLapRecorder will grab some OBD-II data (importantly, RPM) using one of the many cheap OBD-II -> Bluetooth devices out there).
There's similar software for iPhones but (a) I'm not an iPhone user so I can't really make suggestions and (b) it seems there are issues getting fast GPS updates on the iPhone for accurate timing, because Apple doesn't allow arbitrary BT GPS receivers to be used (e.g. see the explanation here:
Compatibility)