I'm talking about http://www.basecamphq.com... the project management app.
It's a great tool but you can't buy it outright, it's always hosted by 37signals. I'm sure there's lots of people who would like to be able to tinker with it to suit there own needs - but you can't.
So how hard would it be to recreate Basecamp? See http://wisdump.com/business/being-37signals-for-free/ for a good article along the same lines...
The component parts are deceptively simple - message threads with comments, ajaxy to-do list, a calendar, multiple login accounts, file uploads - but I still guess it would be pretty hard to put these together into a stable package.
Just a thought - could you do a simple version using WordPress and a bunch of plugins?
Message threads of course are pretty much there already, and people management, and even file uploads to some extent.
What do you think? (It's a crazy idea I know)