This could probably be done better, but this was a first stab.

This is great, cheers! Except I changed the last line to:
$(window).on('resize load', adjustIframes);
Presumably WIN is something from another library you were including?

Yeah, it was a reference to window.

or make it with pure CSS: http://andmag.se/2011/11/responsive-embeds/

This is really interesting. I just tried this with some iframe videos in a responsive theme I am working on and my only issue is that the video poster at very small widths seems a bit distorted. I know this is probably an edge case but this certainly is better than what I had which was nothing. Thank you! :)

Has anyone tried http://npr.github.io/responsiveiframe/?

Works well. We use it at NPR. If you have any problems file an issue. Always trying to make sure we keep up cross browser compatibility.

Can someone help clean this up for me? I tried embedding this into a WP javascript editor for use in a WP site, it doesn't work. I cleaned up the spacing syntax as they suggested, but I still get this:

function adjustIframes()
{
$('iframe').each(function () {
var
$this = $(this),
proportion = $this.data('proportion'),
w = $this.attr('width'),
actual_w = $this.width();

        if (!proportion)
            {
                proportion = $this.attr('height') / w;
                $this.data('proportion', proportion);
            }

        if (actual_w != w)
            {
                $this.css('height', Math.round(actual_w * proportion) + 'px');
            }
    });
}

$(window).on('resize load', adjustIframes);

!!!!!!!!!!!!!!!!!!!!Sorry for how this window is formatting the code!!!!!!!!!!!!!!!!!!!!!!!!

Errors

line 2 character 5
Expected exactly one space between ')' and '{'.
{
line 3 character 9
Missing 'use strict' statement.
$('iframe').each(function () {
line 3 character 9
'$' was used before it was defined.
$('iframe').each(function () {
line 11 character 17
Expected exactly one space between ')' and '{'.
{
line 16 character 26
Expected '!==' and instead saw '!='.
if (actual_w != w)
line 17 character 17
Expected exactly one space between ')' and '{'.
{
line 22 character 1
'$' was used before it was defined.
$(window).on('resize load', adjustIframes);
line 22 character 3
'window' was used before it was defined.
$(window).on('resize load', adjustIframes);
undefined
$
adjustIframes 1
, $
'each' 3

Thank you!

Works great! Thank you very much!

Great job, thx!

Very cool! Thanks!

Nice work, Aaron. Works like a charm.

For a more complete solution that keeps the iframe size to the content when the content changes, or the browser resizes. Check out this library.

https://github.com/davidjbradshaw/iframe-resizer

This is using Jquery? how about not using Jquery? :)