If you like Gmail Chat and you use Opera, there's a simple tip that will make you happy.
Download the latest build of Opera 9 (warning: Opera 9 is still pre-beta). Then go to gmail.com, logout if necessary, press F12, select "Edit site preferences", go to Network tab and select "Mask as Firefox".
Login to Gmail and you'll now have Gmail Chat in Opera.
Update: Gmail Chat works in Opera 9.10, thanks to a workaround in browser.js.