Firefox with Greasemonkey: The Enhance Gmail Greasemonkey script incorporates Google Calendar and the Google Chat gadget directly into your Gmail account. Install it, then click the Calendar or Chat links at the top of the page to open your inline calendar or chat gadget. If you don't like the default setup of the inline layouts, you can edit the script and change a few simple values to get it looking nice (like the screenshot above, which also features Remember the Milk integration).
For example, to place the calendar on the bottom half of your window (which I prefer) rather than splitting the window vertically, open up the Greasemonkey script and edit this line in the top section, changing:
const OPEN_CAL_BELOW_MAIL=0;
...to...
const OPEN_CAL_BELOW_MAIL=1;
Likewise, I adjusted the Calendar height using:
const CAL_HT = 60;
...rather than 70 percent, which seemed to take up too much of the window. In all, though, this is a really great tool for more tightly integrating your Google Apps dashboard. Enhance Gmail is free, works with Greasemonkey and Firefox. Perhaps we'll see some Better Gmail integration in Enhance Gmail's future. Thanks Redd!
Enhance GMail [Userscripts]