For K-5 Students:
Cyberchase
Cool Math 4 Kids
Fun Brain
Primary Games (Math)
Math Playground
Typing Pal
Especially for First Grade:
http://www.softschools.com/grades/1and2.jsp
http://mathforum.org/mathtools/cell/m1,3.7.1,ALL,ALL/
Especially for K-2:
http://www.beaconlearningcenter.com/WebLessons/BrendasBedroom/default.htm
Early Math Practice : http://www.sheppardsoftware.com/math.htm#earlymath
Especially for Second Grade:
http://www.randomhouse.com/kids/magictreehouse/
http://www.enchantedlearning.com/subjects/dinosaurs/index.html