A while back, I made a format that's been really fun. I play it with my Vintage buddies a lot. It basically enables you to get in a round of magic with any group, even if you are the only one with cards...kind of like Cube. Different from Cube, there's no drafting or deck building. It's one 200-card deck that you all play from. Here are the rules, and I'll break down some interesting tidbits:
200 card deck: 30 red, 30 white, 30 green, 30 black, 30 blue, 30 artifact, 20 multicolor/hybrid, 0 land
No more than one copy of any card
No sideboards or any zones other than exile, graveyard, library, hand, battlefield
Any player may only play 1 spell during any turn
Everyone has unlimited mana of all colors at any time (no mana burn)
Everyone uses the same library, graveyard, exile
No X spells that target an opponent (fireball, braingeyser, etc.) or abilities that are pumpable for infinite power (shivan dragon, etc.)
No spells or abilities that directly end the game (Enter the Infinite, "You Win the Game" effects, etc.)
You start the game like any other in drawing 7 cards. There is no mulligan; you keep what you get. Phases are the same and max hand size is still 7. Everyone starts at 20 life. You play until last-player-standing. Every spell is a bomb, from any set. We playfully call this format "Nuclear Magic" because it's often haymaker answering haymaker with infinite mana and one spell a turn. Cast Emrakul on your turn? I shrink him 15 activations of my flowstone overseer. Cast Progenitus? I respond in your EoT with Terminus.
One REALLY fun aspect of this is the shared library and grave. Really unplayable cards become broken. Example: Misinformation - the one-mana black instant that returns 3 cards from an opponent's graveyard on top of their library in any order. Well, mi casa es su casa in this game, so I can EoT stack the next 3 draws to be the best and worst from our shared graveyard. I will redraw that Obliterate you cast, the next player can draw that crappy Cockatrice (It's fun to throw in some dud-ish cards like this given the shared zone interaction), and the 2nd opponent can have that psychatog (now that the other 20 cards from a quickly building grave had already been exiled with it). If you thought Brainstorm was strong now, just wait until you play with a 200 card bomb deck and a shared library! Even a card with cycling can be DEVASTATING as you throw off your opponent's math on their carefully stacked topdecks that they planned to screw you with while setting themselves up!
Abilities get the benefit of infinite mana, so a derpy card like Crowd Favorites instantly goes from your unplayable chaff binder to a bomb that can tap down the battlefield indefinitely until killed!
Commander cards fit well with the multiplayer format, and everything from Alpha to the latest set is legal. No banned or restricted list! You might choose to allow ante cards or not - you own ALL the cards, so you get back the ante at the end anyway. Playgroups will determine what's TOO good, even in a deck where all the cards are busted. I like to keep a side pile of 10 cards per color just to swap cards in and out over time to keep the stack fresh.
Be creative with your stack and dust off the unplayable garbage to bring them to new life. Here are several cards I love to play in my stack (keep in mind - one spell a turn and infinite mana with shared zones!!!):
Hail of Arrows
Sire of Insanity
Master of Cruelties
The list goes on and on! So many "useless" cards reborn!