You can "drop in on" a person, and you can "drop by" a place. For example: "I'm going to drop in on my friend on my way home." "I'm going to drop by the supermarket on my way home."

Ooh! Great question. Brad C gave an excellent answer. You "drop by" a place, and "drop in on" a person, but this is a native speaker nuance, meaning this is a small difference that native speakers (or fluent speakers, etc) can choose to ignore when speaking and still be understood. For example: "We just dropped in to say hello" (=) "We just dropped by to say hello" "You can drop by anytime" (=) "You can drop in anytime" "Joan loves to have friends drop in" (=) "Joan loves to have friend drop by" However, if a friend tells my that they are planning to "drop in," to me, it sounds like they might have been driving or shopping around town and they suddenly decided to come over to my place (or home) for a while. But if they are just going to "drop by," I would think that the visit would be very, very short. They might not even come into the house. So I think when someone "drops in" they usually spend more time actually visiting a person, whereas if they "drop by" the visit (should) be much quicker or more brief. More examples (because I love examples): Husband: Who was at the door? Me: Oh, that was Lauren. She was just dropping by to say hello. (vs) Husband: Who's in the living room? Me: Oh, that's Lauren. She was out shopping and decided to drop in and have a drink. -Again, this is a small thing I have noticed, but other Americans might feel differently. I still hope that this helps!-