MyChron and XT Racing are the main ones. Both are cross-compatible with their beacons.
My main gripe with the old MyChron Lites was that the user interface was not very intuitive. Reliable though.
I have an XT, and the interface is friendlier, but after a year of use, it has proven to be somewhat fragile. Frequently opening it up to re-connect wires/harnesses that have become undone. It's got somewhat of a "electronic kit project" feel too it. Some folks have also said it is not as reliable as the MyChrons (in terms of picking up the signal).
MyChron has a new one out called the MyChron Lite TG. Much better user interface, and seems just as rugged as the old ones. If I were buying new, that's probably what I would get today.
MyChron:
http://www.aimsports.com/
XT Racing:
http://www.cc-2000.com/xtracing/en/