Algorithm and data structure implementations for C programmers
Though I have not used GDSL ( Generic Data Structures Library ) I think it's worth considering as it implements many frequently used data structures.
The Algorithm Design Manual by Steven Skiena
(source: alberton.info)
Another C library worth checking out, especially because it hasn't been mentioned in answers to this question and also the other duplicate questions: the C Algorithms Library, it can be found at http://c-algorithms.sourceforge.net/ and is covered by a BSD-style license, i.e. it can be freely used in any project. I've used it myself in several smaller programs without encountering any problems.