It's probably because chess variants are so easy to invent that there are so many of them. Luckily, I have discovered this extensive collection of chess variant-playing applets. Luckily for my wife, anyway, because I will stop pestering her. I have never been a fan of playing chess versus computers, but some of these are merely interesting, trivial games.
That category would include Dunsany's Chess, Joust Chess, Narrowness, and Peasant Revolt, some of which are games that are almost chess problems. Maharaja and Sepoys aptly demonstrates the overwhelming might of the amazon. It's also useful to have chess for four players, as well as the truly bizarre.
I remain fascinated by the toy chesses: Miniature Chess (5x5), Mini-Shogi, Toystore Chess, Petty Chess (5x6), Speed Chess, Dragonfly Chess (6x6), and the 19th-century Diana Chess. At the least, they seem like interesting ways to teach or learn chess, and games are very short.
There are a few variants inspired by fantasy or fiction, including Alice Chess, Gary Gygax's DragonChess, and Edgar Rice Burrough's Jetan, possibly the first fictional chess in speculative fiction.
Historical forms of the game and their derivations are represented by Chaturanga, Shatranj, Shogi, the circular Byzantine Shatranj, and its contemporary revival as Circular Chess.
Some of the modern chess variants like Berolina Chess aspire to be a serious direction for orthodox chess to go forward in the future. Aside from Fischer Random Chess, the most common variation of chess invented by skilled players for serious play includes two pieces, one that combines the moves of the bishop and knight and the other that combines the rook and knight, and a consequently larger board. These combination pieces date to at least 1617, but there is no consenus on whether to use one or both pieces, nomenclature, castling rules, initial setup, or board size. Capablanca Chess, developed by world chess champion José Raúl Capablanca (and played with Emmanuel Lasker), and Grand Chess are important contenders, although there are several others (1, 2, 3, 4, 5).