ProjectPier is a Free, Open-Source, self-hosted PHP application for managing tasks, projects and teams through an intuitive web interface. An opensource version of basecamp! Looks pretty basic, not nearly as ajaxy / designed as 37 signals.
Collabtive is an open source clone of BaseCamp (the 37 signals project management software) built in PHP backed by MySQL.
Unfuddle is a project management tool, a little like trac, based around subversion / git hosting, ticket tracking, etc. Hat tip to John Berns.
Redmine is a project management / bug tracker / etc written in Ruby on Rails. It supports multiple CVS back ends including SVN, CVS, Git, Mercurial, Bazaar and Darcs.