While the American federal government is shut down one of the many casualties is NASA.gov. There are numerous fantastic resources that are now completely inaccessible (except via use of archive.org, which has some problems with the newer homepage) including information on the past and current missions, images, news and astronomy information. This is a massive shame.
Here’s a list of some sites that might be able to help in the meantime.
Astronomy Picture of the Day – http://www.star.ucl.ac.uk/~apod/apod/astropix.html
Hubblesite – http://hubblesite.org/
Space Telescope – http://www.spacetelescope.org/
Other Space Agencies
Doubtless there are hundreds of fantastic resources I have missed so please let me know of any in the comments.