I just created my first Plugin and like to show it to you now.
It is a Sidebar Plugin wich allows you to check a IP-Adress (with specified Port) over a Socket-Connection if it is online. You can also define that maximal Timeout and a Description.
If your Server is online, it shows also the response-time.
With that you can check different kind of server, I use it per example to check if my Gameserver is online
Demo of the Admin-Interface:

Demo of the Sidebar:

So, here you can find it:
www.xeranon.ch/serendipity_plugin_onlinecheck.zip
So, i would be pleased to hear what you think about ot, what can be done in a better way or whatever.
Ps: I hope I'm right here with this