Windows/Mac/Unix (Firefox): The URL Fixer Firefox extension automatically corrects common URL typos in the address bar.
URL Fixer will change .cmo to com, htp to http, etc. Previously-mentioned (and much loved) OpenDNS offers this handy auto-correction as well, but URL Fixer separates the correction feature from the DNS settings. Also, URL Fixer can confirm corrections before it makes them if you want it to. URL Fixer is a free download, works with and wherever Firefox does.