Haggle is another graph library for Haskell. It aims to support large graphs efficiently and compactly. It differs from fgl in that it does not use inductive graphs. Instead, graphs are built using mutation in the IO or ST monads and then frozen into immutable forms for processing.
This library is more restrictive than fgl, but provides more compact (and hopefully faster) implementations. See the haddocks in Data.Graph.Haggle for much more information and examples.