12x21a all updates:
- Added Ender Chest.
- On SMP, the Ender Chest's contents will be the same for all players on a server. 
- Added Emerald.
- Added Emerald Ore.
- Added Villager Trading.
- Made Buckets stackable.
- Added Snow to Creative Inventory.
- While sitting inside of a minecart pushing the directional keys will make it move the direction you are pressing.
- Only works on rails.
- NPC Villages change style according to their biome.
- Stars are more realistic.
- They are now slightly smaller and slightly brighter.
- Changed Gravel texture.
- Improved Options menu.
- Added Pyramids.
- Added a second Golden Apple made with Blocks of Gold.
- After being consumed, the player receives 30 second Regeneration IV, plus Resistance and Fire Resistance for 5 minutes.
- Added Sandstone Stairs
- Dispensers will eject Water and Lava source blocks (from buckets)
- Will also take water and lava source blocks if there is an empty bucket inside
- Boats will now pass through lilypads without breaking the boat; the lilypad drops as an item instead.
- New command; /seed displays current world seed
- Passive mobs can be easily pushed around by simply walking, most likely indented to help the player navigate through animal farms more easily.
- Removed Server Jar dependency in Single Player.
- Removed old Chest texture, causing Locked Chests to appear purple.
- Emerald Ore has the wrong texture.
- A Ruby like item can be found in the items.png but does not appear in game.
- Loading a world from a previous version corrupts it.
- Generated structures are very buggy.
- New Golden Apple does not appear in the creative mode inventory screen.
- Villager Trading for items worth more than one emerald doesn't work if you place the emeralds one by one.
- Single player still thinks it requires server/minecraft_server.jar. A blank file with the same name can be used.
- When generating a Superflat world, you will get chunk errors - there will be some chunks generated in default, and the other are generated in superflat.
- Pressing F2 to take a screenshot may crash your game and render your world unplayable.
- There are currently quite common lighting bugs. (Server Issue)
- Sometimes when Saving and Quitting to Title, the World Save may corrupt. and you may spawn in a empty area.
- New inventory screen in Creative mode.
- Blocks and items are sorted by category, each category in its own tab. The categories are:
- Building Blocks
- Decoration Blocks
- The inventory can be searched for specific items: just start typing while in the inventory screen. The search results will be displayed in the Search Items tab.
- The player's survival mode inventory is accessible via a tab with the chest as its icon.
- Brewed potions will not be included in their own category.
- To close the new inventory window you must press 'Esc'. Using the default inventory key does not currently work.
- Added Monster Egg (Silverfish blocks) to Creative inventory.
- Saving and Loading chunks is slightly faster.
- Removed Server Jar dependency in Single Player, again.
- Updated language files to correct the diamond/emerald mix-up.
- Emerald Ore is now green instead of red.
- Added a crafting recipe for sandstone stairs.
- Added the new Golden Apple to the inventory.
- Cannot rename any world when playing single player.
- When generating a world or exiting a world, the "saving chunks" and "loading chunks" is absent.
- Pressing F2 to take a screenshot on Creative Mode's Inventory will cause it to switch to the Search Tab.
- Slimes do not always appear to be the size they really are; for instance, a slime that is the size of a tiny slime may have the health, drops, and slime-spawning abilities of a big slime. Any size can appear to be any size.
- English (US) is missing from the language selection screen.
- The on-state Redstone Torch can be found in the "Decoration Blocks" tab, instead of the "Redstone" tab. Meanwhile, the off-state Redstone Torch can be found in the "Redstone" tab.
- When buckets are stacked and you pick up a liquid, all the buckets disappear except one bucket which will be holding the liquid.
- Soul Sand cannot be found in the creative inventory (even with the search feature)
- The /me command is now for ops only in multiplayer
- Chunk errors are very common.
- Mobs that have more than one skin will always be seen using the default skin (such as sheep and villagers). Reloading the world while near the mob can fix this.
- Cocoa bean plants disappear without dropping cocoa when they are hit by water.
Follow-up with more from 12w21a and also 12w21b!
Installation steps for Windows (originally posted by Eglixus)
Help installing the new snapshot (12w21a) for windows:
1) First go to www.mojang.com and download both the client and the server files.
2) Next bring up your start menu and type in the search box %APPDATA% and hit enter.
3) This will open a new window. Open the .minecraft folder and then open the bin folder.
4) Delete the minecraft executable jar file.
5) Now go to your downloads, and copy and paste the new minecraft jar you just downloaded into this folder.
6) Then go back to your .minecraft folder and create a new folder named server
7) Copy and paste the minecraft_server jar file you downloaded into this folder.
8) Launch minecraft and enjoy the new snapshot.
For Mac users, courtesy of TomScheeper. Thank you Mr. Scheeper!
For mac users:
Step 1: download the minecraft snap shot, and the minecraft_server.jar. http://www.mojang.co...apshot-week-21/
Step 2: Open up finder and press "SHIFT+G+CMD" than type in "~/library/application support/minecraft" than you go to the bin folder
Step 3: Paste the minecraft.jar in the bin folder and make sure to back up the old minecraft.jar ( like rename it to oldminecraft.jar)
Step 4: Make this map "~/library/application support/minecraft/server" and paste the minecraft_server.jar in it
Step 5: You're done! Yippee!