I once read a blog post of the creator of the buttons of gmail, and all was custom. What was more surprising for me, was that different teams in different apps (at that time) didn't seem to share UI components like those
I think they use Google Web Toolkit for things like Gmail.<p>Example:
<a href="http://gwt.google.com/samples/Mail/Mail.html" rel="nofollow">http://gwt.google.com/samples/Mail/Mail.html</a>