Minecraft 1.4, AKA the Pretty Scary Update, arrives on October 24. Until then, the 1.4 pre-release is available now for download on developer Mojang's site. Remember, this isn't the public update, so if you'd rather avoid the risk of installing-at-your-own-risk then you only have to wait five more days.
The Pretty Scary Update brings with it a namesake amount of changes and additions, including a Wither Boss, Witch creature, direct access to server mods, anvils for repairing and combining items, and a whole lot more on top. The Minecraft Wiki has a thorough tab on what's popped up in prior snapshots, and so offers an extensive look at what players can expect from 1.4.