RedMine is the latest incarnation of the Trac clones written on top of Ruby on Rails. It is released under the GPL. Much more successful than the late Collaboa (discontinued), it's getting more and more popular.
And there are indeed lots of good ideas that we could now "steal back" in Trac, in one form or the other: multi-project support (#11025), ticket progress bar (#1084, a long time favorite of mine I never got around implementing), ticket comment editing (#454), bulk ticket changes (#525), etc.
And let's not forget ticket dependencies (#31), very important!