What software can calculate aqueous solution equilibria?
ChemEQL is pretty nice in some instances and free, but it looks like it has been mostly abandoned. 

I'm aware of Titrator, but it only runs on Windows and I've never had  a chance to run it. It's also free.

I've heard good things about Visual MINTEQ, but it is Windows only and I've never had a chance to run it. Supposedly well maintained.

There is also PHREEQC, which sort of works on stuff other than Windows. I hear it is harder to use, but probably worth it.