I love game props.
Sometimes I think it's 1/2 the reason that I want to run a game. I've given out dog-eared, tea-stained maps, treasure cards, fake jems (candy), fake gold and silver (italian wedding candy), and all manner of letters and personal correspondence. It's a lot of work and I don't find the opportunity to do it all that often; but it's always been worth it.
NPC cards (either as sheets with pictures, or actual cards) are a great prop at having your party recognize and remember your NPCs. In the case of NPCs the picture is almost more important than the accompanying stats. The Free RPG Blog posted about their own NPC cards and suggests using pictures of famous people. This works great in a current-day or futuristic campaign, but can be more difficult in a historical or fantasy setting. Still, Hollywood is prolific and so pictures from specific movies can provide a lot of familiar faces (assuming you and your players watch the same movies).
Maps are the last game prop. I've been buying the map packs from WOTC -- in a desperate attempt to stop me from ever having to draw a map again. Despite a way-too-tiny table upon which we game, the map is always popular with players. I know a few players who strongly dislike playing without a map; and I know many of my games have been a severe disappointment I'm sure.
I'm in a number of games (as this site shows), and really only one uses props reguarly. One occasionally has a newsletter; but the other one uses cards for everything, gives out little plastic gems for keeping track of power points, and so on. It's great. In the future, I'm hoping to aim for somewhere in between.