Two libraries come to mind. I and others on the Frenetic[0] team have used both at various points in time to implemented automata-related algorithms. The first is a project called DPRLE. You can find its home page and my GitHub clone of the SVN repository below:
The second is a library that I wrote called TDK. It implements a generalization of BDDs, allowing for variables to have a lattice structure and terminal nodes to have something like a semi-ring structure. You can find the GitHub repository here:
Last I checked, TDK is on OPAM, while DPRLE is not. Hope this helps!
-Spiros E.