One of the major EHR systems used in the United States, VISTa, was developed by the Veteran's Administration and is open source. It's sufficiently successful that it's been adopted not just by other government agencies but a number of private hospitals. Most instances still seem to be based on a commercial M implementation, but some use the fully free GT.M stack
It's M based and in general not on the cutting edge of technology, but it's long service life has shown that it fits the need. I've worked with it once at the Indian Health Service and while not exactly Web 2.0 it is quite powerful and familiar to the staff there.
Just to provide an additional data point, a family member who is a physician and previously used this system before his workplace switched to one of the proprietary private alternatives still occasionally laments on how good it was in comparison.
But the datastore itself reminds me of some of those early NoSQL object stores. It isn't JSON, but you can throw anything in there and get it back -- pretty efficiently!
It's hard to table-ize a lot of clinical data - at least with a standard/universal table. GT.M gives you a solid enterprise quality way to store "stuff".
It's M based and in general not on the cutting edge of technology, but it's long service life has shown that it fits the need. I've worked with it once at the Indian Health Service and while not exactly Web 2.0 it is quite powerful and familiar to the staff there.