Tools

Certificate checker - Check your servers SSL configuration

Documentation

Introduction to SSL - Quick overview of what is SSL, keys, chains, and certificate Authorities
OpenSSL common commands and tips - Configuration, testing, and debuging with openssl
PEM files - What are they? why? how to work with them?
Testing my SSL configuration - Checking the validity of your keys and chains

Additional resources

Transport Layer Security - at wikipedia
OpenSSL - SSL Toolkit