There are simply far too many sites of interest to list, so I'm concentrating on just a couple, and specifically to do with cosmology.

The SETI @ home project - I'm including this one, as it's on the boundary between hard science and the "New Age". This is a very serious site that is analysing radio signals from space to look for extraterrestrial intelligence - and you can help. So far, over three quarters of a million people have downloaded their free screen saver, which analyses data for them while you sleep and sends it back to them. The site is sponsored by some very well-known companies.

NASA official web site - this site is vast