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, the only way to connect to a server is to find out the IP of it to connect. There will soon be a list, similar to creative multiplayer.
 * The addition of Snowy and Icy terrain, plus craftable snow blocks.
 * The addition of Boats.
 * Cow, slimes, and chickens mobs.
 * New music.

Upcoming

 * More mob types
 * 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 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.

Currently, the new game mode is under heavy construction and is very buggy. More information can be found on the Known Survival Multiplayer Issues page.

Originally, Notch has allowed several people from the Minecraft IRC channel to participate in the first tests of Multiplayer. The first tests were private. Notch then allowed invitees to invite other people; however the password was leaked and the server was quickly filled up. After that, anyone could get into the test server (when it was online) by typing a password that was shown on the multiplayer screen.

Notch released the first version of the server software on the 4th of August, 2010. The client was updated so that players could enter the IP of a server to join it. As of 10 September 2010, SMP now finally has a server side inventory, signs and chests. Furnaces have also been fixed to work almost correctly. However, Notch said it wasn't secure yet.

Version History
See patch history.