B.2. GNATS

GNU GNATS is one of the oldest open source bug trackers, and is widely used. It's biggest strengths are interface diversity (it can be used not just through a web browser, but also through email or command-line tools), and plain-text issue storage. The fact that all issue data is stored in text files on disk makes it easier to write custom tools to trawl and parse the data (for example, to generate statistical reports). GNATS can also absorb emails automatically by various means, and add them to the appropriate issues based on patterns in the email headers, which makes logging user/developer conversations very easy.

http://www.gnu.org/software/gnats/