The problem with launching a multiplayer shooter is that players are part of the content. Without players, you've only got half a package, even if there is a single-player mode. MercurySteam's Raiders of the Broken Planet (a Gears of War-styled co-op shooter with a light PvP twist) has always looked rather nice, but without people playing it, it's a hard sell, and the game just never got off the ground.
In an attempt to zap some life back into Raiders in time for the next episodic campaign release, for the next two days, you can grab the entire game (including three DLC solo/co-op campaigns) free on Steam to keep forever. Grab it now, and you've got something for your crew to do over the weekend.
From the little bit I've played so far (the tutorial and half a mission solo), it's a very nice-looking game with an aesthetic halfway between American sci-fi comics and European space-weirdness, which makes sense, given that MercurySteam are Spanish. It seems more cutscene/story heavy than most co-op games, and plays a lot like Gears of War, only with automatic use of cover, and a jump button to allow some climbing around when needed. The mix of ranged and close-quarters brawling remains, plus each hero has their own weapons and special abilities.
One interesting mechanical twist is that there are no ammo drops. Similar to Doom 4, you need to punch bullets out of enemies, so there's a real incentive to get up close and use your melee attacks even if you're a snipey character, which gives you good reason to at least try to stick close to your team. The missions seem pretty bombastic as well, with a lot of environmental stuff going on while you're trying to put bullets in heads.
While you can play the game solo, the focus is co-op with a twist. Your regular team of four players get to be the heroes, but anyone that has completed a given campaign as the goodies unlocks the option to play as the Villain Of The Week to keep the hero team on their toes. All the grunts, minions and bosses will be on your side, and while you're not especially durable yourself, you get to add a little bit of human unpredictability to the bad guy forces. Plus, you respawn, while your minions don't. The trailer above gives you a taste of the dynamic.
Claiming your free copy in its entirety takes a little more leg-work than most Steam giveaways, but not too much. The demo version of the game - a tutorial and the first mission - is listed as free-to-play, and you'll have to download that first, run it once (and probably complete the tutorial while you're at it), then launch the game again via the Steam store Play Now buttons in each of the three DLC campaigns in turn, permanently adding them to your account.
You can grab the base Raiders of the Broken Planet game on Steam here, along with the extra Alien Myths, Wardog Fury and Hades Betrayal campaigns. Once they're unlocked, they should no longer be greyed out on the mission selection screen. There's a fifth campaign in the works, which'll eventually cost real money again, but from now until Friday you can grab everything else to keep, which seems like a good chunk of game.