It depends on the age of your kid. Linux and GCompris (http://gcompris.net/-en-) has a great range for a variety of ages, but at a rough guess I'd say almost all for the under 11s and a reasonable amount for the under-5s (my son is nearly four and has been playing all sorts of games on it for probably a year now).
As has already been said, it depends on the age of the child but you could have a look at Timez Attack (www.bigbrainz.com) which is a maths game to help teach multiplication, in the style of "Doom". The ogre-like baddies "attack" the player by presenting multiplication problems and the player fights back by solving them - a correct answer deals damage to the ogre. I think the difficulty level can be adjusted to suit different age groups/abilities.
We use it in maths lessons at the school in which I work and it seems quite popular - I've even seen children playing it in their own time.
I think there is a free version and a more comprehensive paid version.