Here is one interesting software development environment, especially for web applications.
Squeak is an open source implementation of Smalltalk. Seaside is a Web Framework for developing web applications in Squeak. Pier is a CMS built on Seaside. DabbleDB is also built on Seaside — an incredibly impressive web app.
So far, I’ve installed it on my Mac iFive (OS X 10.4.10).
