Using a 'mystery' theme, all the games on offer are centered around, you've guessed it, mysteries. Oh, and there are some detective games too, which I guess is kind of the same thing.
There are a bunch on offer that I can recommend, such as The Max Payne series, Alan Wake, and The Blackwell series. Although Alan Wake is only half price, and I'm sure I've seen it much, much cheaper in other sales. But it's still worth picking up if you enjoy creepy horror games with great stories.
I would also highly recommend the Broken Sword games, the first two at least. I haven't played the fifth one, but I hear good things. However, you may want to check out the offer over on Humble Bundle first, as for just $10 you can pick up Broken Sword 1,2, and 5, along with some other games.
Check out the full list of games on offer (ends 13th February at 10am PST) from Steam over here.