For 48 hours only, the original Fallout game is free on GOG. If you're a newcomer to the Fallout series and have only played Fallout 3 or New Vegas, this is your chance to see how the series got its start. You can download the game here
If you're expecting a game that is similar to Fallout 3, you're looking in the wrong place, however. The original Fallout was developed by Interplay, whereas Fallout 3 and New Vegas were made by Bethesda Game Studios, a different company entirely. The original Fallout is a CRPG, a turn-based game that is much different than the modern Fallouts.
Despite it's differences, I highly encourage you to head over to GOG and download the game that started it all. It is only free for 2 days, so do it quickly.GOG