JIRA is a proprietary enterprise software product, developed by Atlassian, commonly used for bug tracking, issue tracking, and project management. JIRA is deployed within public open source projects and has over 12,000 customers in over 100 countries. JIRA is widely used as a way to manage bug tracking for many large scale open source and public projects such as Linden Lab's "Secondlife", a related project called "Opensimulator" and many others and is widely known in the developer community. This system is especially usefull for large and/or public projects.
Main site - http://www.atlassian.com/software/jira/