For the next day or so, the superb Freespace 2 is free over on GOG. It's one of the best space sims of all time, so it's worth owning, especially since it's absolutely free. But once you've enjoyed all the game's base campaign has to offer, it gets better: Freespace 2 has also some of the best mods ever made, ranging from Star Trek recreations to Wing Commander campaigns and much, much more.
Before you get too far into the nitty gritty, you'll want to get a program called Knossos. It's basically a all-in-one mod launcher made by fans that'll help you categorise and organise everything. The list of compatible mods is enormous, so you won't be short on content. It works for Ubuntu, Arch Linus and Mac OSX, too. A fresh build of FreeSpace 2 Open was released recently, but you should be able to download all of that directly through Knossos.
Once you've got that, you can start getting into the extra content. All of these are available through Knossos, but I've provided direct links below just in case.
Wing Commander Saga: The Darkest Dawn
The older Wing Commander games are still some of the best space epics of all time, but the Wing Commander Saga fan creation certainly does its best to bring that world back to life. There's 70 cinematics. There's walls of text. There's enormous missions. There's an enormous amount of missions — 55 of them. The briefings are animated. The battles are multi-layered and can get dicey real fast. You'll have multi-fleet battles. Small ship-to-ship encounters. Everything you could want from a fan-made Wing Commander remake, minus Mark Hamill, you'll get in The Darkest Dawn.
But what if you want something a little more anime? Naturally, you couldn't have a space game without someone trying to turn it into Star Fox. And so Star Fox: Event Horizon was born, a mod that takes the spirit of the Nintendo classic and drops it right into the Freespace 2 engine.
What happens if you tried to remake an anime-esque visual novel in Freespace 2, complete with the usual space battles? That's the gist behind Wings of Dawn, a fan creation that focuses on a girl named Dawn as she gets thrown into a galactic war between the Hertak, the Cyrvans and the humans in between.
The best way to install the mod is through Knossos, but the link above works too. There's also an 18+/NSFW patch, if you really want to turn the anime up to 11
Another epic along the lines of Wing Commander, but in its own universe, Hellgate Ikeya is a tough and punishing campaign that's right up the alley of anyone who wants tactical, tougher missions in the vein of oldschool space sims.
A total conversion that focuses on the war between the Combine, Commonwealth and Federation factions, Solaris's first episode contains 10 missions that use an updated AI for grander, tougher battles and a reworked combat that's much faster than what you'd ordinarily get from Freespace 2. Worth a look for people who want a space epic campaign that's more action oriented in the vein of modern games.
A 17-mission campaign that takes place 18 months after the Freespace 2 story, you play as a new pilot recruited into the ranks of the GTVA as you battle an insurgent separatist force. The Procyon Insurgency is designed to be a challenging campaign, so if you're after something that follows the vibe and style of the original Freespace 2, give this one a look.
Want the Battlestar Galactica experience in Freespace 2? Then Diaspora is your best shot. It's definitely worth grabbing if you want a more open flying experience — the open cockpit of the MK VII Viper is a nice change from the standard FS2 ships. You can play as both Colonial or Cylon in multiplayer, and the basic storyline where the Cylons have been silent for 40 years is a nice, simple setup for a space epic. Great total conversion for any BSG fan.
To get Freespace 2 for free, head to the front page of GOG here or redeem the game through the GOG Galaxy client.