Medical providers in the USA are identified by a sequence of numbers called the NPI, which is unique per provider. This data is publicly available from here. Under federal law (FOIA act), providers are required to release data of patient referrals from one provider to another. This data is publicly available from here (Credit to Docgraph). This graph database was built in Neo4j, with data from the NPI registry (May 2014 distribution) as the set of nodes with properties, and with data from physician referrals (2012-2013 distribution) as the set of edges.
You can access a plain-text file explaining the entries of the database here (change url).
You can access the database here: Columbia Academic Commons.
A sample visualization is shown below. A sample of referrals from a hospital (NPI 1548387418) and their corresponding nodes are shown. Hovering over nodes provides more information, including NPI and location. Large nodes are organizations such as hospitals, and small nodes are individual healthcare providers. Different node colors represent different specialties. Refer to the taxonomy chart here. The thicker the edge, the greater the number of referrals. The different distances from the center represent relative distances: those closest are located in the same city, those next closest are located in the same state, and those furthest away are located in another state.
The chart is interactive. Drag the nodes around to see their relationships.