I'm pleased to bring you this story: A New Zealand high school converted entirely to an open source software platform, both server and desktop. By doing so, the school has slashed its server requirements by a factor of about 50 - despite a government deal that effectively mandates Microsoft software in all schools.
This was a bold move, since (as we too often see, unfortunately) a national contract with the government means Microsoft gets paid anyway, even though the school doesn't run any Microsoft software.
The school runs Ubuntu on the desktop, and Mandriva on 4 servers: firewall, storage, and 2 virtual host servers (KVM). They also use Mandriva Directory Server to manage the school's LDAP directory, OpenOffice and Google Docs for documents, Moodle for education, Mahara for student portfolios, etc.