Connect up lane sensors and a start switch to an Arduino I/O board for a nice and inexpensive serial timer. Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. Includes the firmware and source code for a 1 to 6 lane timer. There are also instructions on interfacing to GrandPrix Race Manager software.
Micro Wizard, makers of Fast Track timers, offer a serial port timer in kit form, which works with tracks up to 4 lanes. You solder the components together and then build an enclosure to house the electronics. For as little as $40 for the "Son of a Cheap Kit" or $65 for the "Cheap Kit with Serial Interface", it is an inexpensive way to get a good timer without having to worry about circuit design, layout and trying to find the right components. For $20 more, they will do all of the soldering for you. I do recommend the $65 kit over the less expensive kit, since it gives you the ability to use the timer with race management software. The kit with the serial interface works with GrandPrix Race Manager, just as any other Fast Track timer does.
Offers pre-built modules that you can use for timing, one module per track lane. Wire the modules together and build a bridge to hold the modules. There is no computer interface.
Has 2 and 4 lane timer plans which were developed by a group of Electrical Engineering students from the University of Missouri. There is no computer interface.