I frequently tout Christophe Devine’s XySSL library because it implements commonly-used cryptographic primitives in a concise, low-dependency package. I’ve used XySSL as the crypto libraries in many of my research projects.
A while ago the XySSL website disappeared and I became despondent. However, I have recently learned that the same great code is back as part of PolarSSL, at
polarssl.org. Check it out!