Use web applications more like desktop apps with Prism (or Fluid)
Many web applications these days work more like desktop apps and/or are applications that you use throughout your day. Using the applications through your regular browser presents a problem: to get to the application you have to switch to your web browser, find the window that you have webapp open in (or open a new window or tab if you're launching it for the first time), and possibly switch to the tab it is in. The steps it takes to get to an already open webapp vary depending on the current state of your browser and what windows and tabs you have open.
Instead, it would be much nicer if the webapp had its own entry in your operating system's taskbar or dock (or possibly in the tray if you are on Windows). With the help of Mozilla's Prism
you can achieve just that. If you are using Mac OS X, you also have the option of using Fluid
which uses the same rendering engine as Safari (Webkit).
What both of these do is build a standalone executable based on a given URL, title, and icon. This could even be useful for those deploying webapps to end users. I've seen Adobe Air
used for this purpose, but Prism provides an open source alternative to this which doesn't require any additional runtime.
I've been using Prism for my GMail accounts and web-based todo list for a while now, and it makes using those very convenient (especially since these are apps that I use frequently throughout the day). The latest version (1.0 beta) has some support for badges (for example, can be used for showing number of emails in dock icon), but it looks like that is up to the web app itself to implement that (or possibly through an extension or through scripting). If you select "Show status messages and progress" when creating the app with Prism, the app will have to ability to install extensions, though I have not yet been able to successfully install the Firefox extensions I've wanted to.
Fluid, on the other hand, appears to have more features, such as a Greasemonkey-like "Userscripting", but I have had trouble getting it to actually work. For GMail, it tries to open the app up in my default browser when I try to login.