Check out socialmeter. It gives you the link count for any site (any specific URL actually) from Bloglines, Del.icio.us, Digg, Google, Rojo, Shadows, Technorati and Yahoo. It's a bit blunt and it would be useful as a widget (rather than taking you off the page you're visiting and to the socialmeter page) – not to mention if it had a way to actually parse the link sources – but it's both fun an interesting.

Thanks to Paul at Blau Exchange for the pointer.