You might want to look at XULRunner from Mozilla. At a 10000 foot level, the FireFox browser is a XULRunner application (obviously a very sophisticated one, but...). But XULRunner lets you use Javascript and XML to create applications, and the browser window is one of those components, so you once you get your basic window up, you can likely do pretty much anything you want.
Also, depending on the sophistication of your application, there are several "widget" frameworks (like Dashboard on the Mac, Yahoo Widgets, Windows Gadgets) which are basically HTML runtimes as well.