I don\'t ever recall coming across anything for C that\'s as extensive as C++\'s Boost library.
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).
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/
Glib does compensate it partially in form of various unicode, string types. If you add Gobject you could get some object programming even some garbage collection.
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.
or you can look at ccan http://ccan.ozlabs.org/
but no, there really isn't any complete package, glib comes closest
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