Update: URL Fixer was acquired and is now hosted at http://urlfixer.org/
URL Fixer is an extension for Mozilla Firefox that corrects typos in URLs that you enter in the address bar. For example, if you type google.con, it will correct it to google.com (asking first, if you enable confirmation).
How It Works
URL Fixer fixes common typos in URLs entered in the address bar. It will fix errors like “google.con” or “youtube,com”. You have the option of being asked to confirm corrections before they are applied.
With URL Fixer, you can also specify your own set of custom corrections. Add or remove your own corrections in URL Fixer’s options dialog, accessible via Tools » Add-ons » URL Fixer » Preferences.
You can also correct errors as they happen; if Firefox can’t find the website you typed in, URL Fixer lets you mark that URL as a typo, and you’ll never see that error page again.