The last demo that really made me want to spend money on the game was the Half-Life 2 demo. It opens with the beginning of the game and the cutscene that players of the original will immediately connect with, and then allows you to explore City 17 a little and catch the first glimpse of the story. Then you get to go to Ravenholm, which may not have been the best choice for a demo level as it's supposed to be scary, and demo players who go out and buy the full game won't be as scared if they know what's coming. Regardless of that, though, it allows you to sample the majority of the weapons including the gravity gun, one of the focal points of the game. All in all, you get to experience some wonderfully atmospheric levels and play with some awesome weapons while enjoying the gravity gun and killing zombies with random objects. It lasts about an hour, too. I bought the Orange Box about a week later, and of course, wasn't at all disappointed.