www.robotmarketplace.com has a lot of random robot related stuff. A lot of it isn't
FIRST legal, but it's always worth a browse.
This whitepaper has great explanations of the various types of drivetrains, for dog shifting gearbox design the
whitepaper on 188's '04 design is excellent, the
whitepaper on the Killer Bees' four speed is also excellent, if you're into something different the
whitepaper on the Tigertrons three speed ball shifter is also quite good, and for cheap, simple, and light multispeed gearboxes, it's hard to beat the DeWalt XRP transmission, the use of which is detailed in the
NBD whitepaper. (Although the Killer Bees four speed really gives the XRP three-speed a run for its money)