Factor Game	Freeware. A great game that gives drill and practice on the factors of a number, in a most entertaining fashion. Mind you, this game is probably better if you don't use a computer. The paper and counters version has the additional rule - If a player FORGETS to take any of the factors of a number, then the other player can claim them. Both Mac (see you guys aren't entirely forgotten) and Windows.
Product Game	Freeware. The 'inverse' of the Factor Game; gives drill-and-practice on multiplication facts. Mac and Windows.
WinCalc	Freeware. Does high precision integer arithmetic, to thousands of decimal places.