BitBucket is a Mercurial repository hosting service with a project view similar to Trac in spirit. It is a proprietary licensed platform, and uses a cloud that is free for small teams with unlimited private and public repositories.
As they say:
Develop code as a team. Keep all your development in one place, be it issue tracking, documentation or sharing code.
Lots of good stuff to steal there ;-)
Among other things, they use WikiCreole all over the place, the same way Trac uses its own Trac WikiFormatting everywhere. Furthermore, the wiki is itself stored in a secondary Mercurial repository, much like described in TighterSubversionIntegration#Aprivaterepository.