Basecamp

Basecamp is a unique project collaboration tool by the same guys who do Backpack. They also do a couple of other apps called Writeboard and Ta-da List. Basecamp makes it easy to centralize group communication with co-workers and clients. Basecamp makes it simple to communicate and collaborate on projects.
It's elegant, easy, and web-based. You don't need to download, install, or configure anything (no IT department required!). All you need is a web browser and an internet connection.