Firefox with the Greasemonkey extension: Not since Gmail saw its interface overhaul last October has a scripter embedded Google Calendar and Reader into Gmail easily—until now. Two new user scripts integrate both Google Calendar and Google Reader onto the same screen as your email inbox. Here's what it looks like.
Click on the thumbnail to see a full screen of both GCal and GReader below my list of draft email messages.
There are two drawbacks to these scripts: first, if you've got a full inbox (which most of us do), you have to scroll down to see your calendar and feeds. (This is unlike the Gmail 1.0 versions for Calendar and Reader, which included a folder list link which loaded the cal and reader into the area where the inbox lives). Second, they don't work with Google Apps accounts—just vanilla Gmail.
While your inbox doesn't collapse, the embedded Reader and Cal modules do. Here's a a shot of Reader collapsed so you can see the calendar. (Click to enlarge.)
You can also install the accompanying GCal/GReader- Header Removal in IFrames user script, which hides Cal and Reader top frames to save real estate.
If you want to embed webapps other than Reader and Gcal, there's also the Gmail Addons script, which can put any web page into Gmail with a little know-how. However, for just Reader and Gcal, these do a nicer-looking job.
All three user scripts are free downloads and work with Firefox and the Greasemonkey extension. Thanks, Michael!