If you suspect there might be something else going on, like dyslexia or an auditory processing disorder, there is a (somewhat costly) computer program called "Earobics" which can be helpful, but I think an even better option would be music and dance; either formal lessons, casual instruction from a parent, or even the Dance! Revolution game. (I can provide references for why this is helpful, but it's also just fun.)
Most of the commonly available online programs like Starfall, Learning Odyssey, and Reading Eggs moved too fast in our household and I wound up doing most of it hands-on anyway. The LeapFrog videos were great, as are classic Sesame Street, if you can get a hold of those.
For my oldest we wound up using an Orton-Gillingham trained tutor and I was allowed to sit in on the sessions, which meant that for child #2 and #3 I knew pretty much what to do. I'm also more confident about just letting the process unfold on its own.
I'm a fan of playing cards: Blackjack teaches addition. If you bet pennies on it there is also extra motivation. There is a website called 'arcademic skill builders' that lets kids play against other kids (but no chat function) that can be useful for drilling practice.
Also I think another poster meant "cuisenaire rods" as a hands-on math tool.
HTH