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/)
I've never seen anything. Really, C++ templates are the enabling technology for STL and much of boost, and C doesn't have them. I've seen many C-interface libraries for things like image processing, 3D graphics, networking, etc., but never a real general-purpose library like boost. You can find pieces though - threading libraries, string libraries, etc. - that are similar to sections of boost.