Eternal Silence is a neat Half-Life 2 mod that combines a space combat simulator with a first person shooter. Defend your own battleship while attempting to board the enemy’s battleship and destroy it.
There are different classes of ships, from bombers, which are a bit fragile but good at taking down enemy defense ships, to interceptors, which are good at taking down other player-controlled ships, to gunships, which are personnel carriers. You can also choose from a half-dozen soldier classes, with your choice of machine guns, flamethrowers, ion cannons, or remote-controlled railguns.
It feels a bit like someone took Tachyon: The Fringe and mushed it together with one of the Battlefield series. I’ve only played it a couple times so far, but I’m enjoying it a lot, and it’s free if you already own Half-Life 2. Which you should.