Linux in the NHS
Linux is considerably used in the NHS, mainly in invisible server applications. Other major FLOSS applications in use include the Apache webserver, Squid-cache web caching proxy server, MySQL database, Zope and Plone for content management and certain websites, and of course Perl. The first two internet search engines known to be run on the NHS Net were both Ht:/Dig.
MediaWiki is also in use, although its nature is such that it is commonly not visible outside the department or practice that uses it for reference.
Healthcare IT Systems Running on Linux
OpenVista, the Open Source (GPL) variant of the US Federal Veterans Healthcare Administration VISTA, produced and supported by the World Vista not-for-profit organisation runs on Linux, or on other Unixes.
OSCAR McMaster - the Canadian Open Source GP software is in production use, running on Linux.
Open Infrastructure for Outcomes is also worth a look or two.
An attempt to design a healthcare IT system from scratch is currently underway. Designing_and_specifying_a_GP/Hospital_system