MC Server Mon
MC Mon is a small tray application that monitors the status of any online minecraft server, including the number of players joined/motd. It can notify you when:
the server comes online or goes offline,
when a server becomes empty/not empty,
or when it becomes full/not full.
When it notices a change in the server info, it displays a small notification in the lower right of the screen. This makes it great for checking when your friends private server comes online, or when a really popular server has a slot free, without having to have minecraft open.
Requires .NET framework 4.0 or above.