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/)
ACE (Adaptive Communication Environment) is sometimes mentioned. It's not quite an apples-to-apples comparison. Boost provides more "basic building blocks" whereas ACE provides more of an tightly integrated framework geared towards telco products. I have used both extensively and I find Boost vastly superior. One big advantage of Boost is that many of its features will show up in the STL extensions for the upcoming C++0x standard (see, for example, <http://en.wikipedia.org/wiki/Technical_Report_1>).