Big Deck Formats
Elder Dragon Highlander (EDH) is one of the most popular casual formats of all time, second only to general multiplayer.
5-color may be the most intriguing formats I've ever seen. It's a big deck format with a minimum of 300 cards, and requires players to use at least 25 of each color.
Cube Drafting, for when you want to play the very best.
Standard, Extended, Legacy and Vintage.
Even More Wacky Format Resources