CCAN provides two kinds of useful C code: junkcode is a collection of uploaded code with no particular order, and CCAN modules are more structured, with documentation, dependencies and testcases.
You can search, browse summary pages for all the modules or use the Bazaar repository at http://ccan.ozlabs.org/repo.
You can upload a .tar.gz, .tar.bz, .tar or .zip containing C code; if you have not created an account this won't be visible until the contents have been checked by a human.
"GPLv2 or later" and supersets thereof (eg. LGPLv2+ or BSD) licenses preferred.
We have a low volume mailing list for discussion of CCAN in general, and you're welcome to join.
We also have an IRC channel: #ccan on Freenode.
We also have a wiki; feel free to enhance it.