From Minecraft Wiki
Beta 1.6.6 added minor changes and was released to fix some of the bugs and crashes in Beta 1.6.5.
- Added Performance in Video Settings, Options Menu. Can be set to Max FPS, Balanced or Power saver.
- Bone meal can grow tall grass and flowers on grass blocks.
- Glowstone Changes
- Changed the material of glowstone from "glass" to "stone". This means you need a pickaxe to get resources from it. This also means you can now place redstone, torches, levers, etc. on them.
- Made glowstone drop more loot (2-4 glowstone dust).
- Made the recipe sizes for creating wool and glowstone smaller (2x2 instead of 3x3).
- Boat changes
- Boats float up to the surface quicker.
- Boats falling down into water lose their vertical momentum much faster.
3 bugs fixed
- Fixed beds not working very well in SMP.
- Destroying a boat will no longer spawn the player too low, causing them to sometimes fall through the ground.
- ⇧ Shift+clicking a tool into a chest no longer restores it to new.