Sidebar Gadgets let you do more with Gmail that just read email. For instance, you may add items to your TO DO list with the “Remember the Milk” gadget or you may catch up with the latest news using the Digg gadget without ever leaving the Gmail website.
You can post status updates to Twitter, shorten long URLs and more with the help of these sidebar gadgets in Gmail.
To add a gadget to your Gmail account, go to Settings – > Labs and turn on the setting that says – “Add any gadget by URL.” This will add a new "Gadgets" tab to your Settings pane where you’ll be able to specify the URL of any gadget. This gadgets will show in the left column of Gmail.
Once you have saved the changes, go to this iGoogle page that has a list of iGoogle gadgets created by Google. Not all of them will be compatible with Gmail but most should work without issues.
Open an Gadget and look at the browser address bar where the address of the gadget is listed. Copy everything after the “url=” parameter that’s your gadget URL. Go back to your Gmail Settings and paste this address in the “Gadgets” tab.
If it throws up an error, you might want to add http:// before the gadget URL in Gmail.