Plugin development
The Remote System Explorer (RSE) is built on top of Eclipse, and uses the Eclipse user interface, architecture, and frameworks for writing plug-ins. So, in order to understand how to write extensions (more commonly referred to as "plug-ins") for the Remote Systems Explorer (RSE), you first need to understand Eclipse. Eclipse is an integrated development environment that was designed to be extended by tool vendors. The core of Eclipse consists of a platform (the workbench, workspace, UI, etc....) and mechanism for loading plug-ins (OSGi), Java development tools (JDT) and a plug-in development environment (PDE). The PDE is tools inside of Eclipse that all plug-in developers should use for writing their plug-ins.
If you look at the Eclipse web site you will find a lot of other projects that extend this core platform. For example: Data tools, Web tools, testing and performance tools, and modeling tools. It's important to know this so you don't get overwhelmed when visiting the Eclipse web site. The resources section of the web site provides resources for all these projects. So you will find articles on things like "How to process OCL Abstract Syntax Trees" which is just darn scary (and completely unrelated) for developers who just want to extend the RSE. However you will also find articles on "Creating an Eclipse View" which might be exactly what you want to do!
After creating your "Hello World" plug-in, the best place to get started is reading the Eclipse Platform Plug-in Developer Guide. There is also a ton of resources here.
The current release of WDSC (7.0) which is based on Eclipse 3.2.
Here you can find a list of vendors / users you have plug-ins for WDSC WDSC/Plugin.