The open-source FreeCell Pro program with 3 built-in solvers. However, note that these solvers generate very long solutions, 2-4 times longer than the solutions on this site ;-)
Freecell Solver - a program and a library written in C for automatically solving layouts of Freecell and similar Solitaire variants. The author of this solver is Shlomi Fish. There you can find the links to many other solvers also.