We installed Subversion and Trac here last week to replace Starteam for defect tracking and source control. Firefox and GMail/Yahoo/Hotmail are rampant. Asterisk answers the phone. All these open-source software tools have replaced commercial products. Even here in a traditionally Microsoft shop, OSS is starting to mainstream. And we didn't need to switch to Linux to do it. Yes, the Asterisk box runs Linux, but it's just a box on the network, like GMail is a service on the larger network called the Internet. Who cares what GMail servers run; Windows clients can use it happily.
Yes we still use Exchange, Visual Studio, and GotoMeeting.com. But in five years?