While its amazing Skype doesnt do this, this app does it great

I have been frustrated with Skype for years, having hundreds of contacts, but only wanting to get notifications for about 15 of them when they come online. This solves that problem. It could be more elegant in how it delivers notifications. One feature that is nice is "close all" so that when you have been away for awhile, you can close them all at once.

ChevalNoir about Contact Alerts for Skype