TracProject (http://trac.edgewall.org/) is an enhanced wiki and DefectTracking system for SoftwareDevelopment projects. Trac allows wiki markup in issue descriptions and commit messages, to create links and seamless references between bugs, tasks, changesets, files and wiki pages. It is a revolutionary tool, and not just for the reasons mooted. What's cool is trac enables you to use WikiMarkupLanguage in your source names and comments, commit comments, and EngineeringTask descriptions, to provide a tight binding between our languages of analysis, management, and design.
It contains a wiki, TracWiki.
The SapirWhorfHypothesis means the former disconnects between these languages led directly to ChineseWhispers and consequent business disruption. Trac cuts right through the BS as if it weren't there.
Trac is also a superb example of MinimalistDesign that should be an object lesson to tool designers everywhere.
See also TracVsFitnesse, TracTicket, BugTrackingSoftware, RedMine