921 design principles and counting.
37signals is a privately held American web application company based in Chicago, Illinois
They've created som very successful web-based apps which all share the same traits: Ease of use and a relentless focus on the core tasks. Most notably:
37 Signals is also the inventors of the open source web application framework Ruby on Rails which was extracted from Basecamp.
Read more about 37 Signals and their products on 37signals.com and on Wikipedia. Also read about their thoughts on their blog Signal vs. Noise.