As for Vista, I've actually got a Vista image installed in Virtual PC at work now, I just have to figure out how to make widgets for its sidebar.
I worked on a Vista server monitor a while ago.
downloadOriginally, Kitsune on the official forums made it. But it was ugly, and didn't work 100%. I made some changes to it, and made it prettier, though I stopped working on it before I fixed everything. To my knowledge, the only problem with the current version is that if you lose your internet connection, the gadget is unable to re-establish the connection.
After installing, you should have a new folder:
C:\Users\<username>\AppData\Local\Microsoft\Windows Sidebar\Gadgets\CoXServerMonitor.gadget\
gadget.xml: Gadget specification
Monitor.html: Gadget display (specified in gadget.xml, in the <base> tag)
ServerMonitor.js: JavaScript to pull information
You can use
System.Gadget in the JavaScript for additional functionality, such as options settings (System.Gadget.settingsUI), or flyout windows (System.Gadget.Flyout.file)