Does your server say "Outdated Server" when you try to connect to it?
It means that, well.. your server is outdated! You'll have to go to http://www.minecraft.net/download.jsp and download minecraft_server.exe or minecraft_server.jar and replace your existing one!
Be sure to type "stop" in your server console BEFORE you replace the file..
BACKUP YOUR MAP!!!!
Minecraft is going to attempt to convert your map to the new save format.. just in case something goes wrong, make sure you make a backup!
Then you can delete your current minecraft_server.(exe/jar) and place to newly downloaded one in the same directory and start up your server.
You'll see some new stuff on the console like this:
2011-02-22 19:57:22 [INFO] Starting minecraft server version Beta 1.3 2011-02-22 19:57:22 [INFO] Loading properties 2011-02-22 19:57:22 [INFO] Starting Minecraft server on *:25565 2011-02-22 19:57:22 [WARNING] Failed to load white-list: java.io.FileNotFoundException: white-list.txt (No such file or directory) 2011-02-22 19:57:22 [INFO] Preparing level "world" 2011-02-22 19:57:22 [INFO] Converting map! 2011-02-22 19:57:27 [INFO] Converting... 0% 2011-02-22 19:57:29 [INFO] Converting... 1% ...... ...... ...... ...... ...... 2011-02-22 20:02:12 [INFO] Converting... 99% 2011-02-22 20:02:14 [INFO] Converting... 100% 2011-02-22 20:02:15 [INFO] Preparing start region 2011-02-22 20:02:16 [INFO] Done (293614151000ns)! For help, type "help" or "?"
Don't worry about [WARNING] Failed to load white-list: java.io.FileNotFoundException: white-list.txt (No such file or directory) - Minecraft now has a Whitelist option in the vanilla software, the server will generate this file on its own and you can now create a whitelist!
Your map will also be converted to the new format.. the white-list.txt will be generated and you'll be all set!
If the server you play on is giving you this error you will have to wait until the Admins of that server update everything! There's nothing we can do to help if you aren't the admin of the server!