Responsive images round-up

After I wrote Responsive images with ExpressionEngine, I realised that this area of web development is likely to see a lot of experimentation and innovation with new techniques being released on a regular basis. Since writing the article, at least another two methods have come to light that I'm aware of, so I thought it'd be good to collate them all in one place.

If anyone knows of any I've missed, I'd love to know so I can add them to the list.

Update 12 May 2012: Chris Coyier has published a comprehensive round-up of current methods, complete with spreadsheet and breakdown of which solutions would fit different situations better.