Snapshot 13w21b has been released and is available through the launcher.
Changelogs:
To get the snapshot, use the new launcher available in the link below. To return to 1.5.2, simply start the old launcher and play as normal.
Windows: Server 1 Download – Server 2 Download
MAC/OSX: Server 1 Download – Server 2 Download
Linux: Server 1 Download – Server 2 Download
Server files:
Cross-platform jar: http://s3.amazonaws.com/Minecraft.Download/versions/13w21b/minecraft_server.13w21b.jar
Windows EXE: http://s3.amazonaws.com/Minecraft.Download/versions/13w21b/minecraft_server.13w21b.exe