Normally $19.99 and originally twice as much, the PC version of BioShock is on sale for just $4.99. No shipping, no sales tax (except in Washington)--it's a digital download. It's also one of my all-time favorite games. BioShock is like a suspense novel come to life, with you as the main character.
It's primarily a first-person sci-fi shooter, but with pivotal role-playing elements that fit perfectly into the story. I won't spoil one bit of that story, but suffice it to say it's compelling enough to keep you playing all the way until the surprising, unpredictable, utterly satisfying end. It's also genuinely scary in places; I actually yelped a couple times while playing.
You'll need to install Valve's Steam software to download these and other games, but don't let that stop you. And don't wait: these sale prices expire January 2.