Project Gazelle or SysDev
Project Gazelle: Created by the What.cd Team! Running a Gazelle Front-End Similar to that of TBSource with heavy modification and a XBT Tracker (C++) as the back-end.
SysDev: Created by the Infamous System. Running PHP Front-End and a PHP Back-End, it imo is the Next Big Source. After testing it was theoretically capable of 450,000 peers on a $70.00 Server.
So whats the next big source?