Question: I don't ever recall coming across anything for C that's as extensive as C++'s Boost library.

Answer: 
You can have a look at glib. It provides quite a few interesting things like containers, unicode support, threading...

Have a look at [its documentation](http://library.gnome.org/devel/glib/stable/)
or you can look at ccan <http://ccan.ozlabs.org/>

but no, there really isn't any complete package, glib comes closest