
Gambas is a development environment and platform for Linux and Unix systems. It uses a custom programming language, similar to (but not the same as and not compatible with) Microsoft's Visual Basic.
Gambas allows you to put together Qt and GTK+ interfaces with drag and drop ease and connect them to code, in a very similar way to Visual Basic on the Windows platform.
While something similar to VB as a language and platform will probably be unpopular with many Linux and open source enthusiasts, using the Gambas environment is one of the easiest ways to rapidly develop a graphical application for deployment on Linux/Unix systems.

The Gambas IDE bears a striking resemblance, again, to the Visual Basic IDE and brings together source editing, drag and drop GUI design and project compilation and debugging from a single interface.
One of the major attractions to this platform is its support for Rapid Application Development (RAD) and its visual approach to programming. Even if you have only a little programming experience, you can quickly put together a graphical program for almost any desktop environment.

Obviously, a development platform like Gambas isn't suitable for all projects. What I think this certainly has utility for is for very quickly building graphical applications to automate certain tasks, for example.
Gambas offers probably one of the quickest and simplest ways to build a graphical application for Linux. Anyone with a little Visual Basic experience will very quickly be able to pick it up and start developing simple applications.

Quanta is a web development IDE for KDE which comes as part of the kdewebdev package.
For those unaware, an IDE (integrated development environment) is a software application designed to provide all the tools you need in one place when you're developing code. Quanta is, in my opinion, an excellent IDE for web development.
It focuses on being extensible, so that it plugs into the underlying KDE architecture to do certain tasks and allows integration with many other application to extend its capabilities. A good example of this is the support for CVS integration through KDE application Cervisia. From the official blurb:
Quanta Plus is a highly stable and feature rich web development environment. The vision with Quanta has always been to start with the best architectural foundations, design for efficient and natural use and enable maximal user extensibility.

Quanta also boasts functionality such as a project system for keeping all the files relating to one project in one place, full support for transparently opening and saving files on any network protocol that KDE supports, a Visual Page Layout mode for instantly seeing the results of your code in the IDE window itself, impressive syntax highlighting and code completion features from KDE's text system and more.
While designed specifically for KDE and runs best when you can make use of the full integration, it will happily run under any desktop environment provided that you have the necessary KDE libraries installed.
Personally, I am a big fan of Quanta and the integration with so many KDE services and support for particularly PHP development. The only negative point I can think of about Quanta would be that it does have a tendency to slow down after a while and if you have many files open, and can be a little bit of a resource hog. Provided you keep your Quanta environment quite light though, I find it a very productive way to develop for the web and I'd recommend you try it too.