Any block which drops an item other than the block itself when mined.
- Lapis Lazuli
- Nether Quartz
- Melon (capped at 9 melons)
- Clay (Capped at 4, but increases the chance of getting 4)
- Seeds (from tall grass and wheat, but doesn't increase the actual wheat drop itself)
- Nether Wart
- Glowstone (capped at 4 dust, but increases the chance of getting 4)
- Increases the chance of gravel dropping flint
- Increase of dropping saplings and apples from leaves
Fortune works differently for different things. Most notably, at Fortune III you have a 20% chance to get twice as many drops, a 20% chance to get 3 times as many drops and a 20% chance to get 4 times as many drops for Coal, Diamond and Lapis Lazuli.
All others work similarly to Looting on a sword, increasing the maximum limit and therefore increasing the chance that it drops a lot of things, but not explicitly doubling or tripling the dropped amount.