Chess variants

Bughouse.net - The best site I've seen about this popular chess variant.  You'll find Internet bughouse chess info, bughouse & chess software, profiles of FICS & ICC members, and an online message board and discussion group
Home Page of The Chess Variant Pages - a comprehensive collection of chess variants
The Losing Chess Pages - a very extensive page dedictated to one of the most popular chess variants

****

3-D, 4 Cubed Chess - downloadable demo of a chess game on 4x4x4 board
3-D Chess - interesting looking chess board
4 Player Chess - chess for four players.  Looks like a fun variant!
Advanced Chess - regular chess with computer assistance
AISE - Italian Chess Variants Federation.  Pages are in Italian and English.
Beer Chess
Bishops - a four player board game
Bughouse Index - a nice index of other bughouse sites, with strategy and tips
Bughouse.net - large site about this popular variant
Cardinal Super Chess - like traditional chess with a large board and additional piece
Checkmates - a four-player variant
Chess 121 - free demo of the latest version of Chess 121 for DOS or Windows 95
Chess: 3D - A 3d chess variant that can be played online against a java applet
Chessapeak Games Co - seller of several chess variants including several 4-player chess games
Chess 2 - normal chess but with two extra pieces.
Chess Variant Pages - Complete  chess variants.
ChexX - a two to six-player variant played on a hexagon shaped board
Chess demo page for RiscOS computers - the only chess variant version for RiscOS computers
Comprehensive Bughouse (book) - A book on the strategies of bughouse.
d2d4 - ( Bughouse) tips and strategy, annotated games, and a chat room.
Ebenfelt's Bughouse Page - Rules, Strategy, Tips
ErrantFischer's Bughouse Chess Page problems & studies.
EJK personal bughouse index - He plays correspondence bughouse!
Four Player Chess
Global Chess - Chess - Jim - but not as we know it.
Gothic Chess Association - rules and information about this variant
HexiChess - a 3D variant
Hyperchess
Knightmare Chess - A translation of the popular French game Tempete sur l'Echequier, Knightmare Chess is chess played with cards. Each card breaks the rules in a wild and unpredictable way
Loser Chess - with information about this popular variant which can be played on the chess servers
Losing Chess Pages - comprehensive site on losing chess.
Losing Chess Strategy  
NetChess - several chess variants to download
Omega Chess -interesting variant with larger board and additional, new pieces
PI: Team Chess Game - Circular chess game involving multiple players
Pieter Stouten's Shogi Page
PowerChess - Chess on a 8x8 board but with 66 squares
Protest Chess - played like normal chess, but with the unique ability to challenge your opponents move and thus create a new line of attack/defence
Quadchess -  a 4 sided board game that measures emotions, integrity and persuasiveness as well as strategy and tactics
Shogi page (Roger Hare)
Sir Bombalot - A superior alternative to chess, showcased in a 3D viewing plane. Sir Bombalot is shareware software and can be played over the Internet.
Sjeng - ( A computer program) plays bughouse & crazyhouse.
Star Chess
Thief - Bughouse interface.
Trancendental Chess - A chess variant played by correspondence. 
Wild Game 24: Bughouse - ICC based rules and strategy.
Zillions Of Games - Software that allows you to create your own variants and then play them on-line!