Bunny Hood
The Bunny Hood is one of the most useful masks in Majora's Mask. It also appears in the Ocarina of Time, but has no real use except to be a mask. The Bunny Hood helps Link run faster, and is extremely useful when chasing fast paced enemies. It can also be used to beat the Postman's game of trying to stop dead on ten seconds in Majora's Mask.
Update: In Ocarina of Time the Bunny Hood has three functions: 1) sell it to the Running Man for max rupees, 2) prevent waking the Stalchildren of Hyrule Field at night and 3) show it to King Zora to discover you are Hylian before the Deku Tree Sprout tells you so.