12w17a
Official changelog is somewhat non-descriptive (One of the items is "Censored").
Editable books - Book and Quill, crafted using a Book, Feather, and Ink Sac.
When a Book and Quill is made, it can be right-clicked to open a screen that allows you to type the book.
Pressing "Done" will save whatever was typed and allow you to edit the book again later, yet will not "sign" it.
When "signing" a book, it will allow you to choose a title up to 16 characters long and disable editing of the book. The Book and Quill then becomes a Written Book, with the icon changing along with the label.
Wooden slabs for each of the four shades of wood added
These new slabs no longer act like stone
An axe should be used to break them
They are no longer immune to fire and lava
New block IDs 125 for double wooden slabs and 126 for single wooden slabs.
Wooden slabs created in older versions of Minecraft are still stone-like and fireproof and will not change to the new slabs when placed or mined.
Leather is now required to craft Books.
Silk touch can now retrieve Ice blocks and Glass Panes.
Ice no longer turns into water in the Nether.
In the menu, Chat Options were added.
Added chat opacity, letting the chat be semi-transparent.
Added chat disabling, removing the chat for SMP servers.
Added chat colors option.
/help added.
Tools now have infinite durability in creative mode.
Minecart and boats are destroyed in one hit in Creative mode.
Various Bugfixes, some regarding entity collision, the pick block function, and chat.
Livestock, such as chickens, can now be "walked through" instead of pushing the player around in crowded pens
Flying through water in Creative mode is no longer slow
You can no longer have a Bonus Chest in Hardcore mode.
Server Commands now appear in italics in SMP.
Notable bugs (Complete bug list):
All Wooden Slabs appear as the Oak slab when placed upside down. This is purely visual.
The Sound and the Music volume aren't displayed correctly.
Wooden Slabs spend light when placed before 12w17a.