Whew.... You really know how to ask a wide open question. There are lots of answers and all of them are valid. Wizards has a few sets of rules for Multiplayer varients. Other web sites have some rules and variations available but you would need to REALLY hunt and search for something worth while.
Jeblucas offered some really good info on
5Color (Prismatic), and Wizards has some information on rules varients and banned lists here:
http://www.wizards.com/default.asp?x=magic/rules/prismaticRules for
Emperor can be found here:
http://www.wizards.com/default.asp?x=magic/rules/emperorCaviat: There are several variations of this style, and everyone plays it a bit differently.
Here is an interesting varient called
Frontier magic:
http://www.wizards.com/default.asp?x=magic/rules/frontierThere are cool write-ups on this site for the
Skittles and
Type4 architypes.
Highlander (1 of any card max in a deck) has several varients, so rules are very difficult to come by.
Other varients I've heard of (though not necessarily recommended) could include
Pentagram (5 player),
Hand/Land drop (any number of players), and
Tribal (any number of players).
Pentagram is a standard Magic environment and any format can be used for rules, banned, and restricted lists. Players sit in a circle. The 2 parties across from you ar your enemies and the people to your immediate left and right are your allies. Allies CANNOT be attacked until BOTH opponents are eliminated. Adds an interesting twist to the game as each ally has 1 common enemy and is an enemy to your other ally. (Fun with diplomacy)
Land/Hand drop is a game where each player gets 5 lands to start, and once your hand is depleted you draw a new hand of 7 cards. Not cool or recommended, but definately broken fun.
Tribal runs a minimum of 20 creatures from the same tribe (goblins, elves, etc), some minor support cards and land. This format is quite popular, and can be found at some local stores as a major archetype. Not sure about a banned/restricted list for this one, but it could be developed pretty easy.
And, Finally,
Rambo (all for one). Probably the loosest and most widely played version of multiplayer. Big games can take quite a while and aren't for the faint of heart. This is primarily what we play (3-4 people), some games fly while other stall out to a crawl.
Any of the above are a lot of fun to play, and can be altered with house rules to keep them fun. We found a great way to spice up our Magic time by adding Avatars. Some groups allow you to choose your avatar to compliment your deck, we make our selections totally random to add some spice to the game. The Avatars are quite cheep and can be found on eBay and the such quite frequently. There are 32 actual printed versions available (4 sets of 8).
Hope this helps you in some way.