Today, I’ve hosted a BitTorrent Tracker server on AppFog. The tracker server is written in node.js by Emil Hesslow and it works great!
Here’s the tracker: http://licaon-track.ap01.aws.af.cm/ (The tracker server is in Singapore.)
I hope that helps you and more users will use the tracker. (Of course don’t use it too excessively as I only used a free plan.)