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.
Limited Formats
Cube Drafting, for when you want to play the very best.
Repack Drafting
Constructed Formats
Standard, Extended, Legacy and Vintage.
Pauper Extended
Even More Wacky Format Resources