Here are some good resources on the web for working on the System i platform.
IBM Information Center
The IBM Information Center is a well of information. Sometimes it is hard to find the information one seeks because of the mass of content.
The IBM Redbooks are a high-quality documentation delivered by IBM.
Aaron Bartell's site
MowYourLawn.com has some nice resources and infos about RPG and the surrounding System i world.
Application Server for RPG (AppServer4RPG)
This is an open source project which lets your RPG programs talk to Java applications through the use of data queues. It can be found at AppServer4RPG.Sourceforge.net. The project leader is Dieter Bender.
This is an open source site which contains many different programs and service programs, most notably WSDL2RPG, Log4RPG and the underestimated BASICS1 service program. The site is maintained by Thomas Raddatz.