Release date: 2009
Like ICO, Demon's Souls is the predecessor to a much more popular game. And although you've probably heard of Demon's Souls, you may not have bothered to play it, even if you're a fan of Dark Souls.
But if you've been avoiding playing From Software's first masterpiece for fear of it not being as good as its successor, think again, because it truly is. If you've mastered Dark Souls you'll feel right at home in Demon's Souls; if you haven't, it's a great place to start.
And if your excuse is that you don't have a PS3, either borrow one from a friend or go buy one right now just to play this game. It's worth it (they're getting cheaper, anyway).