activeCOLLAB. Open source alternative to Basecamp. I'm certain that they're doing well over a million dollars a year now.

In a presentation, the CEO of activeCOLLAB(Ilija Studen) mentioned that they were pulling in 50k Euros a month, this was back in 2007 *

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* Video Lectures.net(unfortunately it's on Croatian): http://videolectures.net/webstart08_studen_sks/