The company I work for has been expanding its use of Linux dramatically. We've rolled out more than 2000 workstations for our end users at remote locations. And we've just finished migrating from Outlook/Exchange to Zimbra on Linux. It's not free, we use commercial versions of the software, bit it's a whole LOT cheaper than Microsoft, with nearly all the funcionality... Still, the decision to move to Zimbra will wind up saving nearly $250,000 once we finish our fault tolerant configuration.
Mark