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.
This IBM Support document answers many questions about MTIs.
Scott Forstie, Db2 for i Business Architect, has a gists page where he publishes useful SQL examples.
This is an IBM i community web site with a lot of great posts about programming and SQL, with excellent examples.
This is the primary clearinghouse for the latest information on Db2 for IBM i.