At freemedsw.apfelkraut.org an overview is available that lists most (hopefully somewhen all) open source projects for the health care sector. It contains only projects whose usage is not limited and that have recently been active (last news or releases not older than a year).
For each project the name, homepage, license type, supported platform(s) and client type (web-based vs. native) is specified. There is also the possibility to submit new projects via a form.

At the moment the following categories are available: Clinical Research, DICOM Tool, DICOM Viewer/Server, Electronic medical record, HL7 Tool, Practice/hospital management system and Other.

For details see freemedsw.apfelkraut.org or my blog entry “Overview of free medical software”.