I used exactly one module from CCAN in the past - failtest (http://ccodearchive.net/info/failtest.html).

And that's a simple idea, done well (and helps me hit the usually untested code paths).

If I had to still work a lot with C (I mostly write bits wrapped in JNI/Python/PHP exts), I would say that the hashtable, list and INI parser would be bits I would never want to reinvent again.