Java Edition Alpha

Alpha (Survival) is the most recent version of Infdev, that includes new updates such as Redstone and Stair blocks. It is playable both with a downloadable client and at www.minecraft.net/game/ (note: You must buy the game to play)  Attempting to load an Alpha map into infdev (if it includes stairs) will cause it to crash. Infdev levels that were saved earlier can be loaded into Alpha with no problem, and save files from Alpha can be loaded into the infdev preview tool even if they contain new block types.

Development
Minecraft Alpha is a renamed and merged version of the Infdev and Indev development branches, signalling a progression towards the upcoming beta. The renamed versions were announced by Notch on 28th June 2010, renaming Creative mode to Classic, and replacing the existing 'in development' modes with a single version. Although this remains the only version linked to by the official website, old development versions can currently still be accessed here (Indev) and here (Infdev), until they are taken down at a later date (presumably when Minecraft Beta arrives).

The game is updated very frequently, due to Notch working full-time on the Minecraft project since 29th May 2010. Bug fixes and tweaks can happen throughout the week, appearing on the game's development blog, with major additions and changes appearing in 'seecret' Friday updates, leaving the game's players to discover the included new features.

Current
Since the release of Alpha, the following features were added on top of Infdev's base:
 * The addition of Redstone circuits, a logical I/O system including switches.
 * Improved AI pathfinding and spawning.
 * New sounds for caves and mobs.
 * Survival Multiplayer Mode (SMP). Currently this is only private.
 * The addition of Snowy and Icy terrain, plus craftable snow blocks.
 * The addition of Boats.
 * Cow and Slime mobs.

Upcoming

 * More mob types (Chickens and Prefix Mobs).
 * More block types

Multiplayer
Survival Multiplayer (SMP) is a game mode currently in development that is highly anticipated by the community. Many players have speculated on the forums how gameplay will turn out once the mode is released. Speculated gameplay highlights include Player vs. Player (PvP), raiding parties, base defense mechanisms, and cooperative player towns.

Recently, the Notch has allowed several people from the Minecraft IRC channel to participate in testing Multiplayer. Currently, the mode is under heavy construction and was very buggy. There was no PvP (Player versus Player) damage or enemy mobs in multiplayer, and when the server had even one person in a position where land would need to be generated, lag would spike dramatically. Notch plans to optimize this so people cannot grief by simply walking.

there is no way to get an invite, unless you are a notable or familiar member of the minecraft community.

Version History
See patch history.