Zillions ( http://www.zillionsofgames.com/ ) provides many chess variants
freely downloadable, such as Grand Chess (popular!). There are also
several free small chess variants where Zillions plays perfectly. It's good
training! At $25 you can unlock the demo and get thousands of additional
chess variants freely downloadable from Zillions's site or from
http://www.chessvariants.org . However, Zillions Fide-chess (also free)
cannot
really compete with commercial programs because it lacks an opening book.

My own chess variants ( http://hem.passagen.se/melki9/chessvar.htm ) have
been tweaked to play better in the opening, etc., and play a strong game of
chess. You can also play the world's most poular game, Chinese Chess, and
its relative, Korean Chess, which is considerably deeper. Playing chess
variants is essentially the same thing as playing standard chess:
calculating
variants and thinking strategically.

Mats W