See the icon next to my name on the top-right of my blog page?
Green means I'm online, reachable by Windows Live Messenger. Click on the icon and WLM web control will fire up in your browser, ready to use and contact me. Nice and easy, no local WLM installation required. I'll try this presence icon thingy out and leave it right there for some time and see how that turns out. If you want to use it for your site, head this way.
There's also the Windows Live Messenger Presence API, which can be queried programmatically to return user's status. Huh!
More info here.