Krugle

The link I found this through [1] describes it as a 'Google for Source Codes' (sp), but when you digg a little deeper it is actually a lot more than that; it is a very poweful app.

I also think that a lot can be learned from their GUI implementation - very much an ide look and feel to it. I would love to see them implement a VS plugin using the same engine, it would be a very powerful addition to any programmer's toolbox (it already is as a web app).

Check it out: Krugle.com

[1] http://digg.com/programming/The_Ultimate_Search_Engine_For_Programmers