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.