FunbrainandFunschoolare two easy to use web sites that are ful of great activities for students across a range of curriculum areas.  They are generally flash based activities aimed at primary / elementary aged students but there are a few activities that would be suitable for high school students also.  Both sites have a fair amount of advertising on them but  I guess that is what keeps sites such as these growing.  Anyhow be sure to check outFunbrainandFunschoolnext time you are looking for some maths and literacy games for your kids.