ADS: Astrophysics Data System
"The SAO/NASA Astrophysics Data System (ADS) is a Digital Library portal for researchers in Astronomy and Physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA grant. The ADS maintains three bibliographic databases containing more than 9.1 million records: Astronomy and Astrophysics, Physics, and arXiv e-prints."
APOD: Astronomy Picture of the Day
"Contains the largest collection of annotated astronomical images on the internet". Co-created by a university professor and a NASA scientist.
ArXiv.org E-Print Archive
ArXiv (pronounced "archive.org") is a repository for research papers in physics, computer science, mathematics and neuroscience. While many of the papers on the server are high quality, be warned that they are not peer-reviewed. ArXiv does moderate the submissions for relevance and appropriateness, and has recently implemented and endorsement system for some submissions. However, this is not the same as peer-review.
A search engine to locate NASA-funded publications hosted in PubMed Central (PMC). "NASA is using PMC to permanently preserve and provide easy public access to the peer-reviewed papers resulting from NASA-funded research."
NIST Physical Reference Data
The National Institute of Standards and Technology maintains this website, and it is very current. You can find numerical data, reference information and information on papers that have been published relating to the data. Be aware that many of the pages should be printed in landscape mode.
This site is brought to you by the Institute of Physics. The site includes news, resources, company information and job listings. Check the "Latest News" section to see short articles on recently published articles.
SIMBAD Astronomical Database
The SIMBAD astronomical database provides basic data, cross-identifications, bibliography and measurements for astronomical objects outside the solar system.