Some years ago, the only thing that came to my mind when I heard the word "web" was "very simple websites", a bunch of bytes available for browing. Google was not even born at that time and everyhing was focused on desktop applications. If your application was able to connect to the web, then it could update itself in a easy way, nothing else.
The world is different now and ironically it seems to be the opposite of what it was in the past. Instead of bringing things into the desktop, the idea is to take them to the cloud, make them available on the web. This is happening very fast, but sometimes it's useful to have some desktop behavior in a web app, such as offline access to your e-mails.
Why can't we use the best things of those two words (web and the desktop)? Even better, why can't we use our web development knowledge to create hybrid apps that work in those two worlds? In fact, we can, and a lot of people have noticed that.
The first one I can recall is Adobe with Apollo, on March 19, 2007. Apollo, after named to Adobe Integrated Runtime (AIR), is a cross-platform runtime environment for building rich Internet applications using Flash, HTML or Ajax, that can be deployed as a desktop application. A very powerful tool able to create nice apps featuring half desktop half web blood.
Right after Adobe, Mozilla started working on a similtar idea and on October, 2007, Prism was born. It is open source, like all the things Mozilla does, and it splits web applications out of the browser and run them directly on the desktop. Prism development is not fast as Adobe's AIR is, but they are moving forward.
If you were thinking about developing an desktop-and-web application in a easy way, take a look at those options above. I'm sure you will have a lot of heavy weapons available to make your ideas to come true.