Firefox only: Firefox extension App Tabs creates permanent, space-saving webapp tabs in Firefox with the click of a mouse. We've detailed how to set up permanent webapp tabs before, but this extension greatly simplifies the process — already slated for Firefox 4.0.
In our previous guide, you had to install the FaviconizeTab extension along with PermaTabs to shrink tabs, keep them around between sessions and prevent them from being inadvertently closed. With this extension installed, just Ctrl+Click the tab, select App Tab, and any tab will shrink down to an icon only on the left of your Firefox window. You can set it to retain your App Tabs between sessions and protect them from closing.
As we mentioned above, the App Tabs feature is on the roadmap for Firefox 4.0; this extension just gives you the option to use it now. If you give it a try, don't forget to install Better Gmail 2 or Better GReader if you want the unread counts in your tab icons.
App Tabs is a free download, works with Firefox. It's currently experimental, so you may encounter some bugs. It worked without a hitch for me on Windows, but rendered kind of wonky in OS X, so your mileage may vary.