Ah, Know Your Current Events. He used to do that every Thursday years ago, but then it vanished for quite a while. I think he's been doing it again since last summer, but not at any predictable intervals.
You know, Friday night, he did Know Your Local CBS Network Weathermen. No mention of Rich Fields, though.
Maybe the best one of those things Letterman ever did was the night they played Know Your Champion Bowlers. On the first question, he puts up this picture of...some guy with the question, "Name this champion bowler." This being before Dave started letting the players cheat, the guy didn't know the answer; it ended up being "Mike Aulby," who neither I nor the contestant had ever heard of. Then, for the second go-'round, they stick up a picture of Queen Elizabeth II with the exact same question. It doesn't go over that well in text, but it was a riot watching it. :-)