Below are sites recommended from Microsoft.com.
Checking facts in Internet sources is one of the key ways to evaluate them, and Refdesk.com, which stands for "reference desk," simplifies this essential step. Since 1995, Refdesk.com has served as a one-click springboard to many of the web's top dictionaries, encyclopedias, calculators, atlases, news headlines, and search engines. The site also includes a handy Homework Helper section (under the Help and Advice column on the lower right of the page) that provides help in all subjects to students in every grade.
As every parent and student knows, books that are required reading are not always available, or if they are, students may misplace their copy before they finish the assignment. Project Gutenberg to the rescue. This site enables you to download more than 30,000 free electronic books to read on your computer, iPhone, Kindle, Sony Reader, or other portable device in a variety of file formats. You can search by title and author or browse their collection of classic works, many of which are available in audio editions as well.
This site provides help in a number of mathematics-related subjects, including basic grade-school math, calculus, algebra, geometry, trigonometry, and statistics. Practice exercises are automatically graded—and this free site also features a glossary, calculators, homework tips, math games, and lesson plans for teachers.
http://www.sciencemadesimple.com/
Science classes—including the ubiquitous science project—aren't as easy for some to grasp as they are for others. At Science Made Simple, elementary and middle school students can get detailed answers to many science questions, read current news articles related to science, get ideas on school projects, and take advantage of unit conversion tables. Users can also find out if their school's textbooks pass the test.
Ever wanted to know why earthquakes happen? How CD burners work? What the sun is made of? These questions, and many others related to computers/electronics, automobiles, science, entertainment, and people, are all answered at this award-winning website. Simply type a query into the search window or peruse the topics by category. Extras include free newsletters, surveys, and printable versions of all answers.