I've been fond of Growl on the mac as a pretty standard notification mechanism.
Growl for Windows is a project that uses the same protocols to do notification on the windows side.
After a discussion in the forums, someone has finally implemented support for one of the last core apps that was missing for a long time: pidgin.
Previously there was a snarl->growl bridge called gnarly that would notify growl of messages, but it's nice to see that such a popular app is getting some attention :).
W00t for open source :).