Battle for Wesnoth is a brilliant turn based fantasy strategy game for the low low price of $0. I had a period where I just played that game in all of my free time and if you played it, you could see why.
The concept is quite simple. You are the commander of an army from a particular classic fantasy faction (humans, elves, dwarves, orcs, dragonfolk, Lizardfolk, Merfolk being the most common). You then summon units which you move around a hex map across different terrains in order to destroy the other faction.
That is the essence of it but there are many aspects to the strategy of Wesnoth which make it so great. The map is made up of different terrains which have different effects on defence and movement. They are quite intuitive - for example, mountains offer the best defence and take up the biggest number of movement points.
Also, units gain experience with each battle or kill. If they get enough, they can level up and become more powerful. Most units start at level 1 and can go up to level 3. If you are playing a story game then you can carry any surviving units over to the next scenario.
I haven't even got round to unit alignment (chaotic, lawful or neutral) and how the time of day affects their attacks or how the game accounts for different attack types and how each unit has a resistance score for each type. There are several more subtle details like this that make the game a great exercise in strategy.
If all Wesnoth had were scenarios where you pick or create a map, your faction and an enemy faction, I could still play the game for ages. however, it goes much further than that as the game also provides several story based games (campaigns) with interlinked scenarios. The stories are far from the standard 'kill all the opponents' aim and usually have some great plot twists such as the original campaign Heir to the Throne. Each scenario in a campaign offers a some interesting strategy choices. Some scenarios involve you going from A to B. Some involve you you killing a certain number of units. Some involve you surviving a certain number of turns. As well as the official aim, you also have to make sure that your units are getting enough experience so that they will level up for the more difficult, later scenarios.
Battle for Wesnoth is simple to learn yet very deep in terms of strategy, its rules providing an infinite number of units, scenarios and campaigns. It will surely keep you hooked for months.