Recently, I dusted off the old chestnut known as The Battle for Wesnoth.
Before I go any farther: You can get The Battle for Wesnoth for free at http://www.wesnoth.org. (Also, it takes up less than 0.5GB when installed.)
I played The Battle for Wesnoth several years and 2 computers ago. Last year, I downloaded it, with the intent to play it again. Earlier this month, I started by going through 2 of the novice-level campaigns, just to re-familiarize myself with the mechanics. Since it was a refresher, I was probably fine before I was halfway through the 1st campaign, but the campaigns are fun, too.
I'm writing this post for a few reasons, the main ones' being: 1. that I need to get 3 posts done before January's end, per my New Year's resolution, & 2. that The Battle for Wesnoth is a good game and more people should play it.
The Battle for Wesnoth is a hex-grid turn-based strategy game w/ a fantasy flavor and an RPG element. Specifically, units can gain experience by fighting & thus level up to become new, better units. For many units, they can become one of two or three units when they level up, so there's a choice to make there, too. Terrain affects both movement and defense, and time of day affects units based on their alignments. There's a lot more to the game than that, of course, but I don't want this post to get all long and rambly. You can go read the wiki (http://wiki.wesnoth.org) or just download & play the game. As I may have mentioned, it's free. ^_^