Here are some interesting, cool, or useful sites that I've encountered:
A "Reader's Digest" for astronomy. Grad students and postdocs take academic papers and distill them into an interesting article, accessible to anyone with some undergraduate education.
A repository for academic papers that is freely available thanks to Cornell University. The astronomy corner of the site (sometimes called astro-ph) updates daily. All papers here are called "pre-prints", which just means that the authors submitted them to an academic journal, but they have not been peer-reviewed yet.
A blog/wiki for astronomers. This site is aimed at astronomy students at all levels (undergrad and up), providing resources, information, and discussion about various topics.