Oliver Steele (I like the skin of his weblog!) has a very nice article about webapplications, dealing about how you should switch from server-side webapplications to client-side webapplications, thereby enabling a more rich experience on the client-side and reducing the load on the server-side (amongst other things).
One of the frameworks to create such a rich client-side applications he mentions, is Laszlo. This is an impressive piece of work. Try some of their demos. I had to reposition my jaw manually, since this gui-experience is mostly only achieved in native applications or in applications that require a huge install of libraries or components on the client-side. Laszlo does not. It requires that you have (Macromedia) Flashplayer 5 or higher and have Javascript enabled in your browser. Technically it caches about 200K on your PC, but that’s it.
While you are there, click on the link that brings you to Laszlo Mail and have a look at the screenshots. Yes, this is a webapplication.
Why do I tell you about yet another web-application framework? Well, on the one hand, you don’t need anything new on your clients. On the other hand, this is a Free and Open Sourced product.