Basecamp does this well, especially version 2, which integrated with email. With version 2 you could have topics in Basecamp which you could post directly do or email and it was searchable and linkable. It was even possible to post messages which were optionally emailed to clients, with their response becoming part of the thread history. Really the best of both worlds. Unfortunately they removed the email feature in Basecamp 3 (and my company upgraded) so we can't do that anymore, but older projects retain their history.
Yeah, basecamp is pretty good at this. It is a pretty solid tool for building projects where having communications and documentation is intermixed is important.
I didn't realized they removed the email piece... seems like a downgrade. Particularly for projects where you have fairly casual participants.
