NASA Astrophysics Data System (ADS): Primary astronomical source for abstracts and links to research papers.
arXiv: Primary source for pre-print and accepted papers in physics and astronomy.
Mikulski Archive for Space Telescopes (MAST): The primary online open-access repository for space telescope data.
Astronomy Picture of the Day (APOD): Amazing astronomy picture each day with an interesting description.
"Getting into Grad School for Physics": A must read for any undergraduate student interested in becoming an astronomer.
A Guide to Publishing in Astronomy: An excellent read that describes the ins and outs of journals and the publishing process.