Variants and expansions have been gathered by a Belgium site.
Normal RISK, RISK 2210, Invasion of the Giant Amoebas, Secret Mission Risk, and RISK 2210 Mars clones are available as one download in a free PC and network play package at Smiley Productions. A RISK GodStorm clone is also under development.
There are two more variants I would like to see included in an online package - RISK NWO(?), where your normal RISK cards have alternate abilities if you wish to play them that way; and Nuclear Risk, where you may request some of your normal RISK armies as nuclear weapons - usually represented by Cherios! House rules regarding invasions by sea and sea control are also out there.
Another RISK clone with many maps and much stronger AI but more limited variants is Lux Delux.
Of course, by the time you finish redesigning the game and add special pieces you might as well play Axis and Allies or World in Flames.