Basecamp is a project management and group collaboration tool. It was launched in 2004 as 37signals first product. The tool includes features for schedules, tasks, files, and messages.
Basecamp is where the Ruby on Rails framework was derived from. Pricing starts at free for one active project and works itself up in tiers depending on how many active projects you want.
Basecamp was redesigned in 2012; the remainder of this article needs to be updated as appropriate.