0 A.D.
Recently I've been playing a lot of 0 A.D., which is a really cool open-source strategy game based on ancient civilisations and putting them at war with each other. It has singleplayer and multiplayer modes but I would definitely recommend multiplayer if you have any friends willing to play, as it's much more fun than playing against a bot.
Gameplay overview
When you start a game you get some basic resources and citizens, which you then have to use to harvest more resources, train more citizens, build more buildings, etc. Over the course of the game you progress through 3 phases, with each one bringing new structures and enhancements, and at some point you can have a go at attacking your opponent(s) if you think you can defeat them. Eventually there will only be one player's civilisation remaining, so the game ends and that player wins.
/gemlog/