Recently came across this software used to keep hospital records while browsing a site called BigThink. It is called Open-EMR (Electronic Medical Records), a 10 year old project with ~500,000 lines of code, became one of the first PHP projects to become a government certified EHR. OpenEMR is an ONC-ATB Ambulatory EHR 2011-2012 certified electronic health records and medical practice management application. It is written in php and runs on LAMP systems, most of the developers are Linux enthusiasts but the user base for the software is mainly Windows machines.
The website for the application http://www.open-emr.org/
The non-profit organization for the application http://www.oemr.org/
List of OpenEMR's features http://www.open-emr.org/wiki/index.php/OpenEMR_Features
Online demo of the software http://www.open-emr.org/wiki/index.php/OpenEMR_Version_4.1.0_Demo
A presentation about Open-EMR http://www.openaffairs.tv/2011/06/open-emr-presentation-and-discussion-at-portland-linux-user-group/
Download the application and or the source code http://www.open-emr.org/wiki/index.php/OpenEMR_Downloads