I just came across some information on an open source EMR on the EMR Update site. The website is located here. Looks fairly interesting. There are a set of very basic screen shots which gives an idea to the look and feel, but also explains the thought process behind the user interface design.
The design of the UI is based on the classic MVC pattern or the Model-View-Controller design pattern. If you're interested in how to implement the MVC pattern in .NET, look at this.
There is an architecture document, but is seems a bit sparse. It does at least show the organization for the project.
The best part is the data model, which is provides a fair amount of detail. It follows a fairly classic healthcare data model for "Patients", "Orders", "Medications" and "Laboratory" entities.
All in all, it looks like fairly neat project. We need more such open source efforts to make the adoption of health IT more affordable. I hope more independent and highly motivated developers take the plunge and really do for Healthcare IT what they did for personal computers and software in the 1980, and the Internet in the 1990's. We can't allow Healthcare IT to be monopolized by the big companies.