I've published my prototype of a graph database, HGraphStorage, on Hackage. Not to say that this is a database you should use in production! This is just a test bed to play around graph database concepts and low level algorithms for storage and retrieval.
Just releasing it was, ahem, fun, as somebody noticed that it didn't build in a clean sandbox, and then that the haddock comments were broken (it's a bit annoying that a one space error in Haddock markup causes the whole install to fail). So hopefully now it's ready for people to try and improve!
Pull requests welcome!!