Snapshot 13w36b (Minecraft 1.7) has been released.
Mojang Team still strongly recommend that you don’t run the snapshots on an existing world if you don’t want to run into chunk errors. There is a known issue with world structures such as Nether Fortresses not transitioning to 1.7 from 1.6, but they will be bringing out a 1.6.3 shortly to fix this.
To get snapshots, open your launcher and press the “New Profile” button. Call it “snapshots” and check the box saying “Enable experimental development snapshots” and save. To switch to the normal version, you can select it in the dropdown at the bottom left corner of the launcher.
Credits: The Minecraft and Minecraft Realms teams