The renewal of this 2010 question asks for more recent NSF math grant proposals.
A collection of proposals from 2000 through 2015 is at Christopher Bishop's home page.
This site maintained by Dan Margalit has one example of a successful NSF math proposal, and in addition contains resources on how to write such a proposal (notably the sections on intellectual merit and broader impact).
The renewed OP also notes broken links: some of these are on the Wayback Archive: