Gorm (Graphic Object Relationship Modeller) is a GNUstep application for building user interfaces and application objects relationships. Its major features include drag-and-drop creation of GUI elements from palettes, run-time loading of additional palettes, direct on-screen manipulation of GUI elements, manipulation and examination of objects via inspectors, and creation of connections between objects using mouse. It is a clone of the former NeXTstep (currently OS X) "Interface Builder" application.
Squeak is an OpenSource and super-portable implementation of a multi-media capable Smalltalk-80 based object-oriented programming environment. It is written entirely in Smalltalk and has a high-performance VM, created by compiling the Smalltalk VM code into efficient and portable C code. It now uses the Self language's Morphic User Interface but still provides the original MVC GUI as well.
The edit-on JavaBean SDK is a customizable, feature-rich JavaBean that allows developers to add WYSIWYG editing functionality to their AWT- or Swing-based applications. It features an intuitive word processor-like user interface that enables XHTML content authoring with XML markup, and has a royalty free run-time license.
TinkerBell is a application development tool for Ruby Web Dialogs. It can be used to develop GUI and Ruby code and a complete application. It works under rwdtinker, the core application for the rwdapplications. Rwdtinker can install and delete installed applets. This program uses the RubyWebDialogs GUI interface, and can be added to with drop-in applets. There are about 14 applets that can be installed. Context-sensitive help is on the windows and tabs.
PylotLight is a cross-platform, themeable, plugin-supporting system for general computing tasks. It is comparable to QuickSilver, which you may recognize if you use a Mac. Current plugins exist for Web searches, Firefox bookmark launching, filesystem browsing and managing, subversion control, terminal control, and more.