To celebrate the final week of GOG.com's fifth anniversary, CD Projekt RED are giving away one of their own games. Buy anything on the distribution site between now and October 17th, and you'll get a free copy of The Witcher: Enhanced Edition. That includes if you buy a copy of The Witcher: Enhanced Edition, so you probably don't want to do that.
What you might want to buy instead is something from the range of classic EA games that are currently on sale, including Sim City 2000, Populous, Theme Hospital, Ultima, Dungeon Keeper, Syndicate, Wing Commander, or Alpha Centuri. If, though some major malfunction, there's nothing in that list you want, Divinity: Dragon Commander is also on sale at 40% off. There's other stuff too, but you'll have to go and find it yourself.
If you don't already own The Witcher, it's certainly worth getting for what is effectively free. It's ropey to the extreme, takes forever to get into the swing of things, and has voice acting so bad that I had to switch it to the original Polish language version so I could ignore how cheesy it all was. Then there are the collect-'em-up sex cards. Despite all that, it does some interesting things, specifically with its questing and morality choices. If nothing else, it's an interesting curio that leads nicely into the much better Witcher 2.