The original 1-up, the Lost Episodes 1-up, and the Mac 1-up.
The One Up (also called 1-Up, Extra Life, or Extra Man) is an item in Wolfenstein 3D and Spear of Destiny that gives the player an extra life, 25 ammo, and maximises his health. In both games, they are only found in secret areas.
Strategy
One Ups are the easily the most useful item in the game. However, since they are so useful, they should only be used if you have low health and/or low ammo, otherwise the effect is wasted.
Be sure that you do get them before you finish the level, though, as they count toward your Treasure percentage. Failure to get all the One Ups will forfeit your 10,000 points, even if all other treasure is obtained.