Here are some good website resources for students to use for homework help and skill practice:

General Resources
Yahooligans : A great filtered search site for kids.
http://yahooligans.yahoo.com/

Discovery Homework Helpers : A collection of websites for student reference:
http://school.discovery.com/students/

AOL Study Buddy: Includes subject level directories, reference tools, and even an essay grader.
http://homework-help.aol.com/

Fact Monster Homework Center : Lots of resources here. Don't forget to scroll down to the bottom and check out the "tools for school."
http://www.factmonster.com/homework/

Smithsonian for Kids : A nice resource for learning about a variety of topics:
http://www.smithsonianeducation.org/students/index.html

Wikipedia : Online encyclopedia. A nice resource for starters but because of it's changing nature facts should be checked.
http://en.wikipedia.org/wiki/Main_Page

Language Arts Resources
WordCentral : Mirriam Webster's online dictionary. Includes a "daily buzzword" for vocabulary building.
http://www.wordcentral.com/

Rhymezone : A nice source for finding rhyming words, but using the pull-down menu students can also find synonyms, antonyms, definitions, quotations and more.
http://www.rhymezone.com/

Book Adventure "Book Finder": Students can answer a few questions about their interests and get a list of recommended books.
http://www.bookadventure.org/ki/bs/ki_bs_helpfind.asp

Math Resources
AAAMath : Explanations of math concepts for K-8 students. Includes exercises and games.
http://www.aaamath.com/

Aplusmath : Check out the flashcards and homework helpers.
http://www.aplusmath.com/

CoolMath : This one is for middle school and up. A good help for math skills.
http://www.coolmath.com/