Hi,
Not really general astronomy websites, but very handy if you are looking for more detail on something or doing research:
arxiv.org/
in particular
uk.arxiv.org/find/astro-ph
(uk mirror)
Its the arXiv.org e-Print archive from Cornell University
adsabs.harvard.edu/abstract_service.html
The NASA Astrophysics Data System
In many cases this has complete journal articles also.
Both sites are free, though read the terms and conditions.
Cheers,
~Al
  