Open Source Equivalent of TFS?

Question

Is there a combination of OSS which can be tightly integrated to perform the following?:

• Source Control (Preferably distributed, since we're not talking TFS. Currently thinking GIT.)
• Continous Integration (Currently thinking TeamCity, despite being a commercial product).
• Work Item Tracking (Really important for organised Agile development. The main advantage of TFS atm is the ability to relate check-ins to work items. I have no idea re. OSS on this one.)

To be honest, I've answered the first two myself. My main query is: does anyone know of a good work item tracker that can be tightly integrated into Source Control?