The French tax agency claim that by upgrading 80 000 desktop systems from Microsoft Office 97 to OpenOffice.org rather than Microsoft Office XP, they can save €29.3m. Upgrading all those machines to Office XP will cost €29.5m and upgrading to OpenOffice.org will only cost €200 000.
It's quite a claim, but the French tax office has slowly and quietly been migrating more and more of it's software aware from closed source proprietary applications to best of breed open-source alternatives.
The usually pro Microsoft ZD Net UK site has quite a nice article listing all the open source tools used, the partners involved, and even talks about the longer term goal of migrating the desktops to Linux: French opt for laissez-faire Linux.