Duo, A Browser for Viewing Desktop and Mobile Websites Side by Side

Duo is a browser with no settings and no search functionality, specifically intended to show websites in desktop and mobile views next to each other. It’s handy for Webkit development, includes dev tools, and can be launched from a bookmarklet via your browser of choice. You can’t change the user agent, and it doesn’t automatically refresh as you edit webpages with your favorite text editor, but it’s an otherwise inexpensive tool for testing responsive designs on the fly. You can grab it for $4.99 in the Mac App Store.