Math:
Johnnie's Math Page is a great place to practice math skills while having fun! http://jmathpage.com/
A great place to practice Math and other educational skills: http://www.funbrain.com/
Math skills review and very good lessons: www.coolmath.com Make sure you go to your math level- probably "Prealgebra".
Science:
Lessons on many different topics:
SCRATCH is a programing language designed for kids. It uses blocks of code that can be manipulated to make a program. I do not know how to use the program. Games can be kept for your own use, or you can post them on the SCRATCH website so everyone can play.
SCRATCH is FREE! It was created by a graduate student at MIT. Click the link and then click the small "Download SCRATCH Now!" button on the left.
There is also a similar program called ALICE, which was developed by a student at Carnegie Mellon University. I do not know as much about it, except that it is similar to SCRATCH, but ALICE was designed more for storytelling than games.
Language Arts/Reading:
"This large collection of online language arts games will help build skills and concepts for grades Pre-K through 12. Each link will take you to a fun and instructive game on FunBrain.com, an educational gaming site for kids. Your students, no matter what their age, are sure to enjoy these activities!"
http://www.teachervision.fen.com/reading-and-language-arts/games/5831.html
http://www.teachervision.fen.com/reading-and-language-arts/games/5831.html
Social Studies:
National Geographic for kids: http://kids.nationalgeographic.com/
Discovery Channel for kids: http://kids.discovery.com/