hex empire
One of those casual time-wasting online games that got the addiction factor right is Hex Empire, by Meta Sauce. Basically a variation of Risk, it's a strategy game on a generated map of hexagonal fields, where you battle three enemy AI armies. So far so standard, until you realize it's really a game of logistics.
With each turn your armies grow and you get to move five of them, but as you conquer more territory, your supply lines become longer, which means attacking becomes largely a matter of rallying armies to the front. (This problem doesn't really occur in Risk, as you can place your armies anywhere you like.)
The trick, then, is to find the right balance between supply and attack - you need to calculate your campaigns. However simple the game, this feature (combined with endless maps to play on) creates enough strategic options for repeated playing...