Google recently introduced a new search service specifically for source code. Google Code Search "helps you find function definitions and sample code by giving you one place to search publicly accessible source code hosted on the Internet." I played with it very briefly, and it definitely seems to have potential. A FAQ is also provided to help you get started.
CRN published an article covering Google Code Search. It's worth a quick read if you ever have a need for this sort of thing (which, admittedly, most folks do not).
Full links:
Google Code Search - http://google.com/codesearch
CRN article - http://www.crn.com/sections/breakingnews/breakingnews.jhtml?articleId=193104796