RTracker is a ruby-based BitTorrent tracker, meant to be used with What.CD's Gazelle (it's dependent on it's database schema), although it can easily be adapted for any sourcecode.

To install, make sure you have
1) Ruby (I use 1.8.6, although any version should do)
2) Rubygems
   -- MySQL gem
   -- memcache gem
   -- bencode gem ( http://github.com/dasch/ruby-bencode-bindings/tree/master )
   -- sinatra gem
   -- Daemons gem
3) A properly filled out config.yml file

The tracker uses Rack, so you can use any server that supports Rack.
To start it using Sinatra (Thin / Mongrel), `ruby tracker.rb [-p port] [-e environment]; ruby schedule.rb`
To use Apache with Phusion Passenger ( http://www.modrails.com/ ), add this to your config file
<VirtualHost tracker.domain.tld:80>
  ServerName tracker.domain.tld
  DocumentRoot /path/to/rtracker/public
</VirtualHost>