This is a really nifty little site that someone on another site I frequent has put up and I think it deserves a mention here. It lets you view or download the videos right from the aggregation page, but right now it only includes 4 hosts.

Here's the site: http://www.oxytube.com/

Here's the proof that I'm not tooting my own horn : http://www.head-fi.org/forums/f11/my-nifty-little-script-search-download-youtube-myspace-vimeo-etc-307037/