999: Nine Hours, Nine Persons, Nine Doors (Nintendo DS)
It's a really fantastic visual novel, one of the best I've played in a long time, and the room escape puzzles are all really clever and challenging. The way the story unfolds in the true ending is kind of mind blowing, especially in the way it incorporates the branching choices structure common in visual novels. The game also makes really novel use of the DS touch screen for both narrative and gameplay reasons, which I always appreciate.
To get all of the game's endings, you have to play through it multiple times, and you can't get the 'true' ending the first time around. Thankfully, starting a new game on an existing save file lets you skip all the dialogue you've already read, which helps immensely. Unfortunately, it doesn't let you skip the 'escape' sections, which results in you having to mindlessly repeat puzzles you already know the solution to. Also the way to get to the 'true' ending is somewhat obscure and counterintuitive, I had to look it up.
But apart from those issues, it plays great and the story is extremely memorable with some great characters. Used copies are pretty cheap, so I'd recommend anyone with a DS and an interest in great visual novels / adventure games to pick it up.