ICU provides a Unicode implementation, with functions for formatting numbers, dates, times, and currencies (according to locale conventions, transliteration, and parsing text in those formats). It provides flexible patterns for formatting messages, where the pattern determines the order of the variable parts of the messages, and the format for each of those variables. These patterns can be stored in resource files for translation to different languages. Included are more than 100 codepage converters for interaction with non-unicode systems.
GNU libavl is a library in ANSI/ISO C for the manipulation of binary trees and balanced binary trees. It is written using a literate programming system called TexiWEB. By way of TexiWEB, libavl is as much a textbook on binary trees and balanced binary trees as it is a collection of code.
Libmcrypt is a library which provides a uniform interface to several symmetric encryption algorithms. It is intended to have a simple interface to access encryption algorithms in ofb, cbc, cfb, and ecb modes. The algorithms it supports are DES, 3DES, RIJNDAEL, Twofish, IDEA, GOST, CAST-256, ARCFOUR, SERPENT, SAFER+, and more. The algorithms and modes are also modular so you can add and remove them on the fly without recompiling the library.