Can't talk about free games without mentioning this.
Tower defence is a newish genre where you build towers to shoot down incoming enemies. Kind of a simplified RTS.
They all share in common the choice of which towers to build (slow but powerful, fast but crappy, area effect, can-shoot-air) and whether to upgrade or build more smaller towers, and whether to build now or hold off until later.
That's it. The entire game. But it's quite fun. There are plenty of variations, too. Destktop tower defence (link 2) is probably the best of the "Build inside the area the baddies roam through, in order to redirect them" variants, whilst gemcraft (link 1) has probably the best upgrade/campaign-path mode.
There are many many others.
They all share a common flaw that it takes quite a long time to find out whether or not your strategy fails.
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FreeGames/TowerDefence