Mojang have been releasing snapshots of their game Minecraft every week for those whom prefer not to wait for updates
This update process is like a public beta, you play early ideas and give feedback. All you have to do to get in on it is download the latest snapshot as a jar file, replace the relevant file in the install directory which is usually hidden see earlier post, and then play with the new features. The snapshots are available for both client and server, usually if there are any major problems Jens will update a patched version within a few hours to a day later.
The latest snapshot is introducing a new save format name “Anvil” that improves saving and data packets transfer, and prepares the way for some modding features. The new file format changes the order of the blocks stored from xzy to yzx.Though modders do not despair, as Jens has also included some source code for the converter to this new format to help you prepare for the the 1.2 release that will force the new saving format
Note the snapshot will also do this, but a backup of the old format will also be saved in case something goes wrong so don’t worry.
Check out the official post at mojang for link to the snapshot http://www.mojang.com/2012/02/15/minecraft-snapshot-12w07a/.