Astronomy
NASA - This is the website of the National Aeronautics and Space Administration. On this website, you can view images from space, read information about NASA's missions, and access much more.
Sky & Telescope - This is the website of the Sky & Telescope magazine. On this site, you can access and read articles of the magazine. Also, photos are included in articles.
Space.com - According to the website, Space.com is the premier source of space exploration, innovation, and astronomy news. On this site, you can access information about astronomy-related technology, space flights, sky watching, and other topics.
Chemistry
ACS.org - This is the website of the American Chemical Society. This website has information for chemistry professionals and students. For professionals, it provides career information, publications, and several online databases and directories. For students, information about programs and resources are included.
ChemIndustry Search Engine – A useful tool in navigating the Web for resources in chemistry. Set up much like Yahoo, this search engine provides categorized access to different areas such as chemical resources, careers, equipment, and academic institutes. A keyword search is also available if browsing the categories does not help. This site is updated daily.
NIST Chemistry WebBook – “Provides thermochemical, thermophysical, and ion energetics data compiled by NIST under the Standard Reference Data Program. The National Institute of Standards and Technology (NIST) uses its best efforts to deliver a high quality copy of the Database and to verify that the data contained therein have been selected on the basis of sound scientific judgment.”
Organic Chemistry Portal (organic-chemistry.org) - An overview of recent topics, interesting reactions, and information on important chemicals for organic chemists.
WebElements: The Periodic Table of the Elements – Periodic table with a profile and photograph of each element.
Physics
Help For Physics Students – The most useful sections of this site are the physics problems, lecture notes, and online tutorials. Since there are so many external links on this page, the probability you will get a broken link is high, but the sections mentioned above do seem like they would be helpful for physics students.
NASA Astrophysics Data System – “The Astrophysics Data System (ADS) is a NASA-funded project which maintains four bibliographic databases containing more than 3.9 million records: Astronomy and Astrophysics, Instrumentation, Physics and Geophysics, and Preprints in Astronomy. The main body of data in the ADS consists of bibliographic records, which are searchable through the abstract service query forms, and full-text scans of much of the astronomical literature which can be browsed though the Browse interface.”
PhysLink.com – one-stop shopping for physics and astronomy on the Internet. This site offers several different tools including a reference section, question/answer “department,” a search engine for physics and astronomy, guest articles, and daily news.