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

Answer: 
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/>
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>).