If github were open source i might consider it as well but as it isnt i am avoiding product lockin by using indefero, setup was easy and its basically a git version of google code so its familiar for most users.
Well i got a alternative to google code as well, check out indefero http://www.indefero.net/
I actually spent a good bit of time last year finding all the alternatives for google products and have implemented most of them.
I enabled the facebook connect for one month on my startup site and learned alot of things.
First yes you will get alot of signups, but most will never return, never make a single contribution and only came to say they did.
Second I will never do it again. The accounts as I said were of 0 value to the site or the existing users that had been invited. The invited users end up using more services and clicking through different parts of the site. They also care more about it when talking to others versus just being able to say they were there. Hope it helps.
indeed i can please check out MiniG http://code.google.com/p/minig/
its meant to do what you want exactly.
If you have a Duuit! account then its the included email service with smtp, imap, xmpp, and web clients.