Not infrequently, you run into somebody posting about whether or not open-source development can possibly work for games, and usually concluding that it can't. Very likely the poster has played NetHack, but I guess Rogue-likes don't count. But. What about Battle for Wesnoth?

Battle for Wesnoth is a turn-based fantasy game in which you control a set of heroes and armies, building up over time to defeat AI-controlled opponents. A slew of campaign and scenarios in the game itself provide probably hundreds of hours of gameplay, but an active community provides innumerable new mods and campaigns you can download. It's been localized for something like 20 languages, and ported to just about every viable OS still in active use. And it is, of course, utterly free, both in the "free like beer" and "free like freedom" senses; the source code is open and available.

Original, Abstract, Naval Combat

So... Battleship Chess. The destroyers move like rooks, right?

Well, no; don't take the name so literally. Like Chess, this is a turn-based abstract strategy game with surprising depth. Like Battleship, its theme is naval combat. But the gameplay is quite unlike those two games.

Each turn, you may move one (and only one) ship in your fleet, which may then fire; if it ends its move adjacent to a friendly ship, both (or all) ships may fire, so planning your moves to maximize your firepower is useful. Different ship types (battleships, battlecruisers, cruisers, destroyers, and subs) have different movement ranges, armor ratings--and armaments. As you might expect, battleships have huge long-range guns, while destroyers have shorter-range but potentially devastating torpedoes. Actually, the ship stats are quite detailed, almost as if this were a naval sim, which it patently is not.

There's fog of war, meaning you don't see enemy ships until they get within sight range--you are not, as in Battleship required to fire blindly until you hit something. You can certainly do that, as many ships can fire farther than their sight range, but ships also have limited ammunition, so there's a tradeoff involved.