Recently, I have begun tinkering around with the WRT tools for my Nokia 5800 so I can start to develop some handy widgets for my phone (and others).
They even have a plugin to help developing these widgets in Aptana/Eclipse. The only problem is that the documentation online is very bad and incomplete. They information is scattered in various places and no where online can you find a decent set of API docs detailing all of the classes and functions. Whatever you find online is incomplete, at best.
Another developer Jappit pointed me in the right direction to find good documentation, but only available inside of Aptana. I have pulled the WRT help / API documentation from Aptana and uploaded it online.
You can view the WRTKit API Documentation online.
Otherwise, you can download the complete WRT API Documentation for Aptana as a zip file.
I also have uploaded the latest WRT Library (1.4) online as well.
Hopefully, now I can start creating a few interesting apps for S60 users...