NASA Astrophysics Data System (ADS): Primary astronomical source for abstracts and links to research papers.

 

http://adswww.harvard.edu/

 

arXiv: Primary source for pre-print and accepted papers in physics and astronomy.

 

http://arxiv.org/

 

Mikulski Archive for Space Telescopes (MAST): The primary online open-access repository for space telescope data.

 

https://archive.stsci.edu/

 

Astronomy Picture of the Day (APOD): Amazing astronomy picture each day with an interesting description.

 

http://apod.nasa.gov/apod/astropix.html

 

"Getting into Grad School for Physics": A must read for any undergraduate student interested in becoming an astronomer.

 

http://www.amazon.com/Getting-In-Grad-School-Physics/dp/1499732244

 

A Guide to Publishing in Astronomy: An excellent read that describes the ins and outs of journals and the publishing process.

 

http://www.astro.washington.edu/courses/astro482/guide.pdf