Gmail has a feature in Labs called "Add any gadget by URL" that allows users to add iGoogle gadgets (or any gadget that is an .xml file) to the side of your Gmail account. 

In order to install any of these gadgets:

1. From your Gmail account, go to the Labs tab in Settings.

2. Look for "Add an gadget by URL." After enabling it, click "Save Changes."

3. Go to the new "Gadgets" tab under "Settings" and add the gadget's .xml address.

Gadgets to Try

Wikipedia

Address: http://www.google.com/ig/modules/wikipedia.xml

Calculator

Address: http://calebegg.com/calc.xml

Sticky Note

Address: http://www.google.com/ig/modules/sticky.xml

Poly Clock (World Clock)

Address: http://gad.getpla.net/poly/clock.xml

Currency Converter

Address: http://helloworld123---.googlecode.com/svn/trunk/currency-converter.xml

Google Calendar

Address: http://www.google.com/ig/modules/calendar.xml

Google Docs

Address: www.google.com/ig/modules/docs.xml

Click here for more info.