Here are some excellent websites for practicing your multiplication tables:
SuperKids Math Worksheet Creator
Here are some excellent websites for practicing basic and long division:
The Great Martini Company Long Division
Cool Math 4 Kids Long Division Tutorial
Games for multiples/factors/primes practice (1, 2 or 3 player games)
http://www.bbc.co.uk/education/mathsfile (go to games, gridgame, level 2 or 3)
http://www.quia.com/cb/8436.html
http://nrich.maths.org/public/viewer.php?obj_id=5468
Games for fraction practice!
Fraction Monkeys
Basic fraction boosters
National Library of Virtual Manipulatives
Woodland's Junior School Fraction and Decimal Games
FunBrain's Fresh Baked Fractions
Games for converting between fractions, decimals, and percents:
BBC's Fraction Decimal Percent game
For 7th Grade: These sites have interactive scientific notation/power of 10 activities
http://www.321know.com/g8_71fx1.htm#section3
http://www.onemathematicalcat.org/algebra_book/online_problems/mdd_powers_of_ten.htm
http://www.edinformatics.com/math_science/scinot6.htm
http://www.sciencegeek.net/Activities/scientificnotation.html