Endless Legend and Wizard with a Gun are both free to play right now on Steam.
The former is a completely free to download for the next week and keep forever, while the latter is only playable for free for the next two days.
Endless Legend is a rather excellent 4X turn-based fantasy strategy game from the creators of Endless Space and Dungeon of the Endless. It features tons of replayability with randomly generated worlds and quests, and you can set the size, shape, topography, and more to create your world, and control every aspect of your civilization.