If you like Google Reader, but would prefer to keep it from taking up an oh-so-important tab in your browser, you'll definitely like GReader Popup. This experimental Firefox extension pulls up a version of Google Reader that sits atop any page you have open. You can plow through feeds using all the usual keyboard shortcuts you would in Google Reader. Then, when done, you simply click the icon in the bottom right hand corner of your browser and it goes away.
No fuss, no tab switching, and most importantly--more real estate in your browser for extra tabs.
It's a wonderfully simple tool, and built off the same idea behind GCal Popup, which does the same thing but for Google Calendar. With both installed you can hop to specific Google apps from any page, using the bottom part of your browser like an application launcher, or by learning the two-button keyboard shortcut that pulls it up.
My one qualm is that it does a hit or miss job at resizing Google Reader to fit your browser. I had parts of the interface cut off, even when running it on a 1600x1050 screen. If you're trying it out on a small laptop or Netbook this can be a deal-killer.