RESTscape doesn't impose any constraint to the tools a developer must use for building services, so use whatever you want.
For site building we use Maven2, which gives a consistent look & feel to the site, and is easy to use.
If you are planning to work with Java, it will be a lot easier for you if you develop with Eclipse + Maven2 + JUnit, as a lot of the initial efforts have been done with these tools.
You can access the source code at the CVS location:
The CVS global structure is:
+--site # Main RESTscape project site
+--java # Java section
+--libs # Foundation java libraries for services
+--commons # "commons" library for java libs and services
+--abstractservice # Base library for easy RESTscape services building in Java
+--...
+--services # Java implementation of RESTscape services
+--file # File related services
+--rdf # RDF related services
+--...
+--php # PHP section
+--... Each subproject (lib or service) has its own site and will follow the same location rules that CVS, except for the main site that will hang from the root path. This will allow easy linking from the main site to each subproject.
If you want to contribute to this project as a developer, just request being added to the project, or share your contribution with some of the current developers.
You can contribute more than just code: documentation, translations,... whatever is welcomed.
If you need a new feature just request it using the SF project tools. As this is for free, no one should expect to have their requested features inmediatly implemented (or ever). If it's vital for you and it doesn't get implemented, consider "donating" to the project... that should work :P