Aliens vs Predator free on GOG till Friday
http://www.gamespot.com/articles/get-aliens-vs-predator-free-from-gog-for-a-limited/1100-6422983/Available until Friday, the classic FPS is also a way for GOG to test their Galaxy multiplayer client. In a press release, GOG VP of online tech wrote, "We're ready to take in more gamers, and AvP seemed like a natural choice: it's high up on our community wishlist, and it's definitely one of the all-time best FPS titles. It also allows for testing Galaxy in a more dynamic environment."
The press release notes that "Galaxy testers will soon be able to play AvP with their Steam friends using GOG Galaxy's cross-play feature, as the game is scheduled to receive a dedicated cross-compatibility update on Steam."