I already know about Arkham Horror, Elder Sign, Gears of War, Ingenious and Red November. Are there more?

Oh, I also know about Lord of the Rings LCG. What have I missed?

I've found that most co-op style games often play well solo as well. I've played the Lord of The Rings Silver Line (now standalone secition) Board/Card game solo, and it works ok. Space Hulk: Death Angel The Card Game plays really well solo. I would say that Death Angel is probably my favorite game to play solo. But, like I say, almost any co-op game works decently.

Here, here for solo playable games.