You may be interested in checking out Carnegie Mellon's The IrYdium Project, which has been developing a Virtual Laboratory, as a free (with a lowercase "f") simulation of acid base chemistry, including thermodynamics, strong/weak acids, redox, indicator chemistry, solubility, etc. It's currently being used by a number of universities and high schools, and is funded by the NSF.

(Of course, in the interest of full disclosure, I should also mention that I'm the author of said software -- having worked on it for the last six years -- so I may be a little biased. :)