WolframAlpha - calculator and so much more.
As read by the public
Astronomy Picture of the Day
Portal to the Universe
Sky & Telescope
New York Times Space & Cosmos
As read by astronomers
ArXiv astro-ph, the research astronomer's news
source, updated each day at 8 pm EST.
Astronomy and related science blogs
Cosmic Variance - astronomers and physicists blogging
about astronomy, physics, and other topics that interest them.
Bad Astronomy is actually about good astronomy and
is written by Phil Plait.
Research-based Experiences for Non-experts
Sloan Digital Sky Survey projects