Infoworld has a short piece by Bruce Perens explaining some good uses of open source for business.

Contract or internal development is very good for your differentiating software because it lets you keep it all. Probably 10 percent of the software in your business is differentiating. Maybe you should consider open source for the rest.

Bruce Perens makes a compelling business case for using open source.  Apache and qmail are some of my favorites open source programs.