Update: Here is a pre-release of Minecraft 1.6.3. You can select these versions in your launcher, or download the server links below.
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. To get the pre-release, do the same but manually select “1.6.3″ from the version dropdown list.
Download Minecraft 1.6.3 Pre-release:
Cross-platform jar: minecraft_server.1.6.3.jar
Windows EXE: minecraft_server.1.6.3.exe