After PayPal leaked (opens in new tab) that the Steam winter sale ought to start tomorrow, I guess GOG felt that we needed to warm up our wallets first. Wouldn't want the debit card to be too hard to remove from its pouch, after all. Today, GOG's entire library of Bethesda games is 75% off (opens in new tab), and most of those games are timeless classics.
Almost nothing on the list is over £2/$2.50, including Fallouts 1, 2 and Tactics (opens in new tab), various Quakes, Elder Scrolls (opens in new tab): Battlespire and Redguard, the Doom series, and Wolfenstein 3D. At the pricier end there's Morrowind. If you purchase any of those, you'll get The Elder Scrolls: Arena and Daggerfall thrown in.
Given how enormous many of those names are, I'm sure you'd be hesitant to associate with anyone who doesn't own the lot already, but in the event than you do, the sale will run until December 29, 4:59 AM GMT.