Basecamp is a Web-based project management tool from 37signals. Groups can come together and work on large or small projects, sharing the same collaborative space.
It's essentially a mash-up of various productivity tools, including a to-do list maker, shared storage space, message boards, and calendaring. What has made it so successful is how it's been tied together. It's well known as a service that fits both personal and group organization into one handy tool.
Basecamp has several tiers of service, with varying degrees of shared file storage and active projects.
Winner: Basecamp (BasecampHQ.com)