A very simple plugin that uses the FitVids.js jQuery plugin by Chris Coyier & Paravel to make embedded videos resize in fluid web designs.
Try resizing your browser window and you’ll see the video resize.
The plugin simply loads a version of FitVids.js that hooks onto the .hentry CSS selector that is automatically put on posts and pages in WordPress when the <?php post_class() ?> function is used (which most themes do).
Demos (try resizing your browser window):
Download: FitVids.js for WordPress.
Full installation details and latest version are available on GitHub.