Iam building a application that helps web developers design website's.It would be appreciated if you(web developers) could tell me what you want to see in a web application.

The only thing I use is a text editor with some sort of text highlighting, at this moment bluefish. (And off course an image editor, GIMP for me). If you want feature's to include in such an application look at the feature list of Dreamweaver, that is ought to give you inspiration.

But what are you trying to build? A web application or a desktop application? (I assumed the latter, but since the title is "Web app"

Maybe he wants to build a CMS. In that case, have a look at Mambo or PostNuke.

You can do a great step by adding new buttons which add tags to his pages when he click on.

its a desktop application to create websites, etc. like dreamweaver