Winter mode

"Winter Mode" is a randomly occurring map type for Minecraft Alpha. It was added on July 9th of 2010 and is the first biome to appear in Minecraft. While being random by nature, a player can force his savegame into Winter Mode by activating the SnowCovered parameter in "level.dat" using a program such as NBTEdit.

Differences
The first difference is falling snowflakes. These snowflakes will create Snow tiles on any tile that is directly exposed to the sky, provided it's solid. Unlike Grass, Snow will not "grow" onto tiles under cover. You can place a heat source (e.g., a torch, fire, etc.) next to it in order to "melt" the snow. Snow can be broken by hand or with tools, and a shovel can be used to yield snowballs, which can then be thrown or crafted into snow blocks. Snow will automatically break if a block is placed on it and will hover in the air if placed over a half block. Snowballs can be thrown at sheep to shear them, or at other Mobs, giving a small punchback without actually causing damage to said mob.

The second unique element is Ice. When a map is generated, all exposed water blocks are frozen into Ice, which is semi-transparent and slippery. Ice can be changed back to a spring tile by hand or with a pickaxe if speed is preferred. Like snow, it can be melted by being placed in proximity to a torch. Neither snow nor ice are affected by redstone torches.

Another difference that is not confirmed is that cacti and reeds do not exist, since any blocks required to grow them would be covered with snow.

Notch has announced that he will make snow tiles break when fallen hard upon. He has also said that Winter mode may become area-dependent, or even seasonal.