libtai is a library for storing and manipulating dates and times. It supports two time scales: (1) TAI64, covering a few hundred billion years with 1-second precision; (2) TAI64NA, covering the same period with 1-attosecond precision. Both scales are defined in terms of TAI, the current international real time standard.
LibKMid is a library that implements features to play MIDI events on MIDI devices. It also includes easy-to-use functions if all you want is play MIDI files in your application (for example, background music in a game), and a console MIDI player. The API is well documented, and the design is very simple for the most common uses.
libwayne is a library of reasonably abstract datatypes and algorithms written in C. They have a consistent user interface. Many of the algorithms are taken verbatim from textbooks, but are coded well. The library contains data structures and code for binary trees, stacks, queues, combinatorial analysis, bit strings, graphs, dynamic arrays, event-driven simulation, priority queues, about half a dozen different O(n log n) sorting algorithms, ODE integration, long long integer I/O, matrix and vector operations, random number generation, search algorithms, and elementary statistics.