Trac
"At the core of Trac lies an integrated wiki and issue/bug database. Using wiki markup, all objects managed by Trac can directly link to other issues/bug reports, code changesets, documentation and files. [...]
Having a network of links between issues/bugs/tasks, code changes and wiki text makes the big-picture perspective of a project truly accessible at any time, and it becomes easy to quickly get up-to-speed on the ."