Gizmos - from Explore Learning
Power Lines - a wonderfully challenging site from the Oswego, NY school district
Rock Hopper - have your frog jump from rock to rock as you practice your basic math skills
Railroad Repair - practice your decimals!
Walk the Plank - see if you can make your teacher walk the plank by correctly figuring out compound words!
Starfall - something for everyone, but just a bit easier
ThatQuiz.org - test yourself on math and other subjects