It is developed by facebook to manage repo's, track and audit changes, be a wiki, help with diff's and code review, track bugs and generally be a swiss army knife of dev team tools.
It looks really good too and it is pretty simple to set up and run. I put it on an Ubuntu server, installed LAMP and a couple of dependencies and a short while later could integrate it with AD and log in. First linux web server I have ever set up. Makes me feel pretty pro! I have just set it up for testing at work, it may or may not end up being used.
I think it is the best thing facebook have done.
Anyone else seen/tried it? Are you using it?