Remember UFO: Enemy Unknown? No? Well, you most probably remember it as X-Com then, and no matter what it was called, it remains the best tactical, turn-based, sci-fi game with strategic elements ever created. It has also inspired Xenowar, a truly stunning Android and PC offering you can download for free over at xenowar.net, though I believe you'd want to grab the mobile version over at the Android Market instead. The game is brilliant, visually unique, addictive and impressively deep.