GOG.com is currently heaving under the pressure of thousands of gamers trying to take advantage of their latest special offer, that is copies of Fallout 1, Fallout 2 and Fallout Tactics, all absolutely free (and giftable if you already own them).
All you need to do to get the games is have a GOG.com account (or register one before the promotion runs out) and add them to your account. The special offer marks the start of the online retailer's Winter Sale (if memory serves, last year they did the same with Torchlight).
In the sale players can select/vote for one of three boxes, with each containing an unknown special offer for the next day, and a variety of other games are discounted. Still, the headline is undoubtedly the three free Fallout titles. Customers new to the digital retailer will also get a selection of other free games if they sign up (including titles such as Beneath a Steel Sky), so that's potentially 15 new (old) games for first time customers.