Single click User-Agent switcher extension for Chrome.
Add extension to Chrome: https://chrome.google.com/webstore/detail/user-agent-single-click-s/igafjdcboeahmnojhkaplghloibnjkbg
you need to switch often between TWO User-Agents. You will switch between normal and custom User-Agent that you can configure in plugins options page.
you need to switch between MANY different User-Agents. There are nice plugins for that already.