The RadRails guys set up their own Trac-like ticket system written in Rails: Hedgehog
Currently they use it for developing RadRails and the software itself. I’ve heard they plan to release it as open source.
I had a look at it and it looks promising. But there are some things that they have to work on. E.g. currently I’m not even able to create a new ticket, but I think they will fix that soon.
Maybe it will get an lightweight alternative to Trac and Collaboa.
Do participate!