[Mystalic]

I'm a fan of Basecamp by 37 signals (basecamphq.com) - It's good for small and big businesses, file sharing, messages to clients, whiteboards and task lists, etc.

Trac is also good because of its integration with subversion and it being open source.

[theshady85]

Big fan of Basecamp. Been using it for 3 years across multiple projects.

[zacharydanger]

I'm not a fan of Basecamp. It lacks the analytical aspects of project management.