Ocelot, their custom-made C++ tracker is a nice piece of code.
They were forced to drop out php-based and XBTT-based trackers some years ago when the server load from their became huge peer swarm became unmanageable.
PHP wasn't really the limiting factor, it's the architecture that XBTT uses that really kills you - an SQL query for every single announce! (With millions of peers that means hundreds of SQL queries per second just to maintain your swarm)
Some other cool trackers to look at are Shadowolf (although I believe that's discontinued, sadly) and Lioness if that's floating around anywhere (What.CD likes wildcat-based codenames)
TorrentFreak made a great story about this[1].
[1] http://torrentfreak.com/what-cd-debuts-lightweight-tracker-f...