SuperEmbed.js detects YouTube, Vimeo, Vine, VideoPress, DailyMotion, and more embedded videos on webpages and makes them responsive. Essentially, this means they strech to fill their container while still maintaining the original aspect ratio.
Respontent is a jQuery plugin that automatically makes your user generated content responsive.
The demo includes both a YouTube and Vimeo iframe embed, both at different aspect ratios. Both fluid, both 100% width.
FluidVids.js tested in Chrome, FireFox, Opera, Safari, IE7, IE8 and IE9.