Chrome: Chrome has some nice quick-access icons on its extension bar, but it only works if extensions put icons there. If you want to toggle other extensions on or off, these Chrome add-ons will help you do it from the address bar.
Certain Chrome extensions — Greasemonkey scripts are a great example — don't put icons on the extension bar, so if you want to enable or disable them, you have to go all the way to the extensions page and do it manually. Quick Enable and Disable are two Chrome extensions that let you do it with a simple keyword in the Omnibar. Just type enable or disable and start typing the name of an extension — it will show up in the auto-suggest drop-down and you can toggle it right from there.
Quick Enable and Quick Disable are free downloads for Google Chrome. Unfortunately, they're two separate extensions at the moment, so you'll have to install both for full functionality.