There are many Munchkin shirts - each with a different special power.
Official Tournament Rules
If you are wearing an official Munchkin shirt (on your torso, right side out), you may draw one extra card from either deck when you start play or when you come back from the dead.
If you are wearing an official Munchkin shirt it may give you special powers besides the ones listed here - see the official site for specifics.
Shirts autographed by Steve Jackson, John Kovalic, Greg Hyland, Andrew Hackard, or Wil Wheaton have additional powers - see the official site for specifics. (Note that signatures may only be valid for certain shirts.)
Wearing two or more Munchkin shirts at the same time does NOT give the bonuses from both shirts. The one on the outside rules.
Use of the Cheat card overrides Rule 4 and lets you benefit from two shirts. (It doesn't matter which one is outside).
But under no circumstance is there any bonus for MORE than two shirts.
It is NOT an official tournament rule, but if you wish to make a house rule that a "Lose Your Armor" card requires you to remove your shirt(s), we cannot stop you.
List of Shirts
T-Shirt
Associated Game
Christmas T-Shirt
Munchkin: Waiting For Santa
Cultic Combativeness T-Shirt
Munchkin Cthulhu
Dungeon Dementia T-Shirt
Munchkin 6
Fairy Dust T-Shirt
Munchkin Fairy Dust
Four Horsies of the Apocalypse T-Shirt
Munchkin Apocalypse
Heart of the Anomaly T-Shirt
Star Munchkin
Lovely Loot T-Shirt
Munchkin and Munchkin 2
Momentous Unmasking T-Shirt
Super Munchkin
Munchkin Sweatshirt
Munchkin
Munchkin T-Shirt
Munchkin
Munchkin Apocalypse T-Shirt
Munchkin Apocalypse
Munchkin Fu T-Shirt
Munchkin Fu
Munchkin Zombies T-Shirt
Munchkin Zombies
Munchkin 10th Anniversary T-Shirt
Munchkin
Polo Shirt
Munchkin
Star Munchkin T-Shirt
Star Munchkin
Unnatural Axe T-Shirt
Munchkin 2
Utterly Unsinkable T-Shirt
Munchkin Booty
Zombie Santa T-Shirt
Munchkin Zombies
OtherPart of the Munchkin series