Here's a fun idea for a five player multiplayer game. First, place five lands - one of each type - in a hat/jar/bag/rabid rat cage and have each player select one. Whatever land they draw is the colour they get. At this point everyone goes home - okay, that's how we did it.
Each player goes to their own little part of the world and builds a monocoloured deck to try and defeat the opposing colours (Green tries to kill Black and Blue, Red tries to kill White and Blue, etc) and then everyone sits down. The game goes as normal except each colour can only attack and use targetted effects against their enemy colours. When one player has no survivin enemy colours he is the winner.
Some guidelines for deck construction to keep it even:
1. No colour hosers (Karma, Lifeforce, Red Elemental Blast, etc)
2. No Protection critters - that would make it TOO easy.
3. No artifacts - let's keep the playing field level.
4. each player has the same number of cards - you can decide before you pick colours.
5. Global spells are allowed and can be lots of fun under the right conditions.
The game has some interesting strategy involved - For instance:
You're the Red Player. You need to take out both Blue and White. Black just killed Green and White is down to 3 life. You have to kill Blue before White or Black will win automaticaly. It gets pretty interesting after a few hours of play...
"...Okay, that's a 20 point hurricane! Tie Game!"