Celebrating its ongoing Spring Sale, GOG.COM is offering XIII free to keep for those who add it to their libraries over the next XLVIII or so hours. This first-person shooter was released in 2003 featuring stylized cel shaded graphics to emulate the comic book upon which it is based and the voice acting of Fox Mulder and Batman. Here's a trailer with a look and here's more:

The President of the United States of America has been assassinated and you're the world's prime suspect. The FBI, the CIA, and a gang of murderous killers are trying to hunt you down. Stricken with amnesia you remember nothing, not even who you are. You're on your own and there is no one you can trust. All you have is a safe deposit key, a mysterious tattoo, and a gun. Clear your name, solve the mystery, and catch the true assassin before he strikes again.

• Exciting espionage story and fast paced action
• Top-notch voice acting preformed by actors such as David Duchovny and Adam West
• ‘Cel-shaded’ graphics and comic book like narration brings the experience of the acclaimed comic book to life on your computer