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/)
Depending on what exactly you're trying to do there is APR (The Apache Portable Runtime library) which is what the Apache http daemon is built on 

<http://apr.apache.org/>

For GUI, there is wxWidgets (formerly wxWindows)

<http://www.wxwidgets.org/>