Portal

One third of the new content in the Orange Box, Portal is too short to be worth the price by itself, but it's one of the most inventive and enjoyable games in a long time for the few hours it lasts. It has a simple setup; you're a test subject being evaluated as you solve puzzles using an experimental weapon that can create a portal between two different places. At first you can only create one end of the link, but before long you can set both locations as you use them to redirect energy balls, cross pits and fling yourself over obstacles. It's a bit mind-twisting at first, but it doesn't take long for you to get familiar with the logic behind it and understand how it works as simply as any other game mechanic. As you go on, the environment gets more dangerous (including turrets that are both deadly yet cute) and the puzzles require more thought to navigate. It climaxes very well with a final encounter that tests everything you learned in a tense situation. It's cool how they were basically able to have a final "boss" without resorting to giving you actual weapons besides the portal gun. They do a good job of shifting between simple but pressurized puzzles and more cerebral ones, preventing your brain from getting tired with the same thing over and over.

You can't talk about Portal without mentioning GLaDOS. She's the artificial intelligence that monitors your progress and tells you what's happening as you work your way through the lab. She seems nice at first, but it isn't long before you realize something sinister's going on. Her voice acting and dialogue is hilarious, and the game's great writing and presentation is half of why it's so good. Few games are actually laugh-out-loud funny, but Portal is one of them. The fact that something horrible is happening becomes more and more obvious, and it's all dark in a very silly way. The line "The Cake is a Lie" is already embedded in the collective consciousness of everyone who's played it. There's also a very memorable and funny song in the end credits that keeps it going. The ending is a cliffhanger and they've already tied Aperture (the company whose laboratory you're running around in) into Half-Life's story, so I wonder how the future of Portal will pan out.