Risk

CleverMedia: World Conquest
A Shockwave version of a classic strategy game.
http://clevermedia.com/game.cgi?worldconquest

Conquer Club
Free multiplayer variation of the board game. Offers player rankings, alternative maps, individual and team games, and a community.
http://www.conquerclub.com

Dominate
Offers moderated gameplay, a ranking system, game documentation, and a discussion forum. [Membership required for some features.]
http://www.dominategame.com/

Global Combat
A variant where turns are taken simultaneously. Offers a player manual, rankings, and online gameplay.
http://www.globalcombat.com/

LandGrab
Web-based strategy game with many extra features such as custom maps, teams, leaders and fortresses.
http://landgrab.net/

Warfish
An online, by invitation only Risk based game which uses a dynamically created image for board display.
http://www.warfish.net/

World at War
Play this turn-based game with 22 maps, including Risk's classic map.
http://www.wawgame.com/