As web apps become more powerful, more popular, and more full-featured, they're starting to replace desktop apps for many people. A Mac app called Fluid can pull those web applications onto your desktop and turn them into OS X native site-specific browsers.
Fluid has a lot of advantages compared to running web apps in your browser: you get a Cocoa app with its own Dock icon, automatic unread badges for sites like Gmail and Google Reader, and built-in userscript support. Keeping your web apps in a separate browser also means that they can't be taken down when another site crashes in some other tab. You can even create menubar apps, so your favorite webapp will be close at hand, right at the top of your screen.
To find icons for your Fluid apps, I recommend checking out the Fluid Icons Flickr group. The users there have come up with attractive icons for most of the apps on my list. Chris Ivarson has also designed a handful of great icons for Fluid apps.