If you have not tried Chess960--aka Fischer Random Chess--then you are missing out. Chess960 randomizes the starting piece layout, but otherwise castling and all the normal rules of chess clearly apply. Both ICC and chessCube offer it, the prior in the aforementioned 'pool' (pairs players automatically, disallowing players choosing opposition, that is to say, avoiding upper or lower rated players).
It forces the issue on positional assessment early on, and sharpens tactics, marginalizing the inescapable and ever present rubric of openings and opening tabiya's. Wow. Very strong medicine.
I just spent a night playing, and at 1300 to 1400 960 rated, 80% of my opposition were dudes with 2000 blitz, bullet, and 5 0 ratings alike... its like 'chess dying'. 5 wins, 16 loses 1 draw= 21. Jesus fu_ck. Try it out. Its like a chess final exam.