First up - Nasa's Astronomy Picture of the Day. THis is from a couple of days ago - and its actually a video of the Northern Lights. Very nice video:
Aurora 2012 from Christian Mülhauser on Vimeo.
Next, One of my favorites - This is a HiDef video compiled from actual photos taken when the Cassini mission passed through Saturn's rings.
5.6k Saturn Cassini Photographic Animation - Expanded, Improved from stephen v2 on Vimeo.