It's Thursday, which means it's time for another exciting Minecraft snapshot! This time around, Mojang has added a few unexpected features to Minecraft. The biggest addition is a new gamemode designed for multiplayer game servers, called spectate mode. The new gamemode is identical to spectator mode in other multiplayer video games like Team Fortress 2, you can fly through blocks, can't access your inventory, and you are invisible.Other improvements include a new invisible solid block to place minecart tracks, redstone dust, and other blocks on top of, powered activator rails will now cause the minecart to shake (and possibly throw you off!), and some bug fixes. You can grab the snapshot by downloading it from the Minecraft launcher. Enjoy!
Source: Mojang
UPDATE: Mojang just released a bug-fix snapshot fixing 20 bugs relating to spectator mode, Java-related runtime errors, and crashes. If you already have the snapshot, the game should update automatically.