Just found this list on Google's Gmail Blog:
There is a powerful but little known Gmail feature that lives in Labs called “Add any gadget by URL.” Once you turn it on, you can add iGoogle gadgets (or any gadget specified by an .xml file) to the side of your Gmail account. While most of these gadgets are built by third-parties and not owned or maintained by Google, they can be super handy.
To install any of these gadgets, follow those steps: 1. From your Gmail account, go to the Labs tab of Gmail Settings. 2. Look for the Lab “Add any gadget by URL.” Enable it, then click “Save changes.” 3. Go to the new “Gadgets” tab under “Settings” and add the relevant .xml address.
Wikipedia Look for a specific query right from Gmail.
Google Calculator Make some quick calculations while typing an email.
Note Add a sticky note to the corner of your Gmail account.
Remember the Milk If you’re a fan of this task management system, accessing all your “Remember the milk” notes from right within Gmail can be super handy.
PolyClock Gives you the time of day for any place in the world.
Currency Converter A real time currency converter.
bit.ly URL shortener Lets you shorten URLs in a single click.
Finally, you might be familiar with the last two gadgets, since they are also available as individual Gmail Labs:
Google Calendar Displays your Google Calendar agenda right from Gmail.
Google Docs Gives you quick access to your most recent documents.
Find many more on this page