I've had decent success using the AdBlock Plus add-on for Firefox.  Also, there's a "Webmail Ad Blocker" plugin from Google that's specifically designed to disable at least some of the (crappy/irritating/malware-ridden) text ads on Google's pages.  Of course, installing these add-ons will cause your browser to "opt-out" of almost all ads on the web, which is very nice, but punishes owners of web pages who are decent and responsible about their ad revenue.

I would hope that Google would realize that they need to make their ads less obnoxious, but until then...