opentracker from Dirk Engling powered one of the biggest BitTorrent trackers, The Pirate Bay tracker.
Atrack is a high performance open source tracker designed to run on Google App Engine.
BitStorm is a small tracker written in PHP which does not require a database server and runs on any PHP compatible web server.
BitStorm 2 uses a MySQL database instead of flat file to improve scalability.
BitTorious is an open source, commercially supported tracker with integrated web-based management portal.
Hefur is a standalone BitTorrent tracker written in C++, under the MIT license.
Ocelot is a BitTorrent tracker written in C++ for the Gazelle project.