Science Portal Suggest a link

General science sites

Google Scholar – Look up citations and papers

Courses and OpenCourseWare

MIT OpenCourseWare – Links to lots of free course materials at MIT (100)
OpenCourseWare Consortium – Many OpenCourseWare sites linked into one (100)
OEDB – Open Education Database. You can search all the open courseware sites to find what you are looking for. Awesome! (100)
edX – Lots of opencourseware courses. (98) Lots of opencourseware courses for fee or free. (98)
Yale Open Courseware – Few good courses (95)
UC Irvine OpenCourseWare – Few good courses (95)
Carnegie Mellon Learning Initiative (95)
Udacity – Lots of technology based courses for several hundred to several thousand dollars. Nanodegrees.(94)
The Great Courses – DVD sets of some interesting courses. Usually not super advanced. Good if you can get them when they go on sale a few times a year. (93)
Udemy Lots of courses usually for a fee. Goes on sale sometimes for 10-20 bucks a course. (93)

Physics Links

Physics Forums – Nice forum for physics and science (100)
Physics Stack Exchange – Questions and answers to any physics question (100)
Complete Irodov Physics Problems and Solutions – Nice solutions set (100)
Sixty Symbols – Physics and Astronomy Videos (99)
Ortvay Physics Contest – Hardest physics test on the planet. (96)
NIST Fundamental Constants – Fundamental constants of physics from CODATA and NIST (95)
US Physics Teams Exams – Several qualifying exams for the US physics team. (92)
Physics and Math Tutor – You can find some nice problems sets and information on tutoring in London. (90)

Math Links

Art of Problem Solving – Where high school students go to prepare for Math Olympiads (100)
Numberphile – Lots of interesting Math topics (100)
Math Contest Collection – More than 20,000 mathematics contest problems and solutions (98)
Mathematics Stack Exchange – Answers to any type of math problem hard or easy (98)
Kahn Academy – About a thousand YouTube math videos at all levels (97)
The Calculus Lifesaver Video Course – 48 hour free video course on calculus. (96) – King of YouTube Math Videos (95)
Mathematical Association of America – Professional site for Mathematicians. Links to books and journals (94)
Paul’s Online Math Notes – Full Notes on Calculus 1,2,3, and differential equations (90)
Wolfram Math World – Great resource for formulas and Mathematica (94)
Math TV – Good math video site. (90)
Cool Math – Nice math site for kids (90)
The Primes Pages – Cool information about prime numbers (90)
S.O.S. math – Some notes on all types of math subjects (90)
Math Tutor DVD – Lots of tutoring videos is various math and science subjects (88)

Chemistry Links

Chemistry Stack Exchange – Questions and answers in chemistry (100)
Periodic Table – Dyanmic periodic table look up properties of elements (100)
NIST Chemistry WebBook – Look up properties of many chemicals (100)
Named Reactions – Named chemical reactions with mechanism (99)
Periodic Videos – A video for each element in the periodic table (98)
Chemistry Olympiad Test – Past Local and US National Exams. (98)
ChemSpider – Chemical property and spectra lookup as well as articles and purchasing (96)
A.C.S. – American Chemical Society (95)
ChemGuide – Tutorials in different subjects of chemistry (94)
Chemtutor – Good for high school chemistry (93)

Biology Links

Biology Stack Exchange – Questions and answers for biology (100)
USABO – Past USA Biology Olympiad questions and answers.(95)

Problems and Solutions – Solutions to the available GRE Subject tests (99)
All Science Olympiads – Good resource for all science Olympiads (99)
Chegg – Solutions for many textbooks (96)
Physics GRE Theory Notes – Good set of notes (95)