Is there any development tools for as400?
Yes. The current Windows-based IDE is called "Rational Developer for i" and you can find more information at http://www-01.ibm.com/software/awdtools/developer/rdi/. The green-screen tools are PDM, SEU, SDA, etc.
Also there is a relatively new non-IBM solution called RPG Next Gen (http://www.rpgnextgen.com/).
If you'd really like a niche career, I'd also recommend Delphi for the AS/400:
You can use the native stuff (the "green-screen" tools). Most of them are also available as client-server (aka GUI interface) too.
You can use a popular alternative as Lansa (www.lansa.com).
Or you can use Java!!!. The iSeries is a hell of Java machine. Everything for your new world impressing application is already on board (JVM, JDK, class libraries (JtOpen) to access the local iSeries stuff). And last, but not least, the world-class database manager DB2 is native in the OS. How about that!?
(and no, I'm not a IBM representative. Just a happy RPG coder ;-)