Glib is a fairly interesting C library, because it has a lot of convenient, portable functions, especially those that handle strings (I llllove those, C string handling is really painful without this lib).
(I originally whined about the lack of documentation here; however, it seems that only the bundled texinfo file is incomplete, the gtk.org site luckily has a complete reference.)
Here's some sort of list of the categories of other neat services and data structures this library gives. (Again, if something was left out, blame the folks who left glib undocumented =)
The GObject object model has been in it since 2.0. (Thanks, RPGeek.)
While it is associated with GTK+, and for large part part written by the same people, it's an independent library and doesn't need X or anything. It is distributed under LGPL.