PUB400.COM

PUB400.COM provides free access to an IBM i 7.5 partition for educational purposes. Anyone can request a user profile for access. You can use this partition to improve your IBM i skills in general, program in CL, RPG, COBOL, and SQL, and learn web technologies.

RPGPGM

This is an IBM i community web site with a lot of great posts about programming and SQL, with excellent examples.

IBM i Open Source

Visit this bitbucket repository for authoritative documentation for IBM i Open Source Software.