This one started in a coffee shop in Palo Alto, CA, way back in 2003, and is now competing with Microsoft Exchange. In between, they raised $30.5 million in venture funding, and created a vibrant web community.
Zimbra has about four million users, and even Bill Gates had to admit that "they've done a good job," even if Zimbra "doesn't even come close to the things that Exchange does."
For more meat on Zimbra, click here, here, here, and here.
Download the Zimbra Colloboration Suite here.
Jay, from Bangalore