LibTomCrypt is a comprehensive, modular, and portable cryptographic toolkit that provides developers with a vast array of well known published block ciphers, one-way hash functions, chaining modes, pseudo-random number generators, public key cryptography, and a plethora of other routines. It has been designed from the ground up to be very simple to use. It has a modular and standard API that allows new ciphers, hashes, and PRNGs to be added or removed without change to the overall end application. It features easy to use functions and a complete user manual which has many source snippet examples.
Libgcrypt is a general-purpose cryptographic library based on the code from GnuPG. It provides functions for all cryptographic building blocks: symmetric ciphers (AES, DES, Blowfish, CAST5, Twofish, and Arcfour), hash algorithms (MD4, MD5, RIPE-MD160, SHA-1, and TIGER-192), MACs (HMAC for all hash algorithms), public key algorithms (RSA, ElGamal, and DSA), large integer functions, random numbers, and a lot of supporting functions.
Botan is a C++ library of cryptographic algorithms. It includes a wide selection of cryptographic algorithms (including AES, DES, SHA-1, RSA, and DSA, among many others). It also support X.509 certificates and several common certificate-related formats and protocols. It has a high level interface, is easily portable to most systems and compilers, and includes a substantial tutorial and API reference.
SILC Toolkit is a software development toolkit which provides full SILC protocol implementation for application developers. The SILC Toolkit is especially targeted at SILC client application developers who would like to create new SILC clients. This toolkit, written in ANSI C compliant source code, is cross-platform and fully supports the SILC Protocol version 1.2. It features a set of libraries, providing support for several ciphers (AES, Twofish, Cast-256, Blowfish, RC5), hash functions (MD5, SHA-1, SHA-256) and public key algorithms (DH, RSA with PKCS#1 v1.5).