http://apod.nasa.gov
Well thanks to the US govt shutdown, even several govt website domains have been shut down, including nasa.gov. The APOD website is hosted on this domain and so, is also unavailable. Can you imagine?
There's an unofficial-official facebook group where you can connect with APOD and other people for your astronomy photograph fix in the meantime.
https://www.facebook.com/AstronomyPictureOfTheDay
edit: it seems that some websites have been removed, whilst others still seem to be "up", but wont be updated, case in point, the mars jpl website: http://mars.jpl.nasa.gov/