Memorable old three-way shooter Alien vs. Predator (released in 2000) is being given away for free on GOG until Friday.
There's a slight catch: it's being given away in order to attract people to participate in the GOG Galaxy beta - the GOG-style online platform that's a bit like Steam.
So to redeem the game you need to sign up for the next stage of the GOG Galaxy beta, and you'll receive an activation code "within the next few days". And once the game has been added to your account, you can keep it forever.
GOG Galaxy will power AVP multiplayer, and a point both GOG and AVP developer Rebellion are keen to point out is that it will be cross-platform with people playing on Steam.