

StreetPass, and I think all DSes could do it. Yes, it tracked steps, and it detected other DSes. If you both had the same game (or related games, such as Pokémon Diamond and Pokémon Pearl, I think was the twin game — I only had Diamond) and you both had your consoles sleeping (i.e. in low power mode), certain things could happen. If you didn’t have the same game, it still registered the connection.
DS users could also anonymously chat with one another. Having a DS at an airport, hotel, or convention was a lot of fun because you’d have tons of people in the area, and if you were playing something popular like Pokémon or Animal Crossing, you were also getting a lot of contacts.

Mine was an R4, but I’ve heard of the Supercard. I don’t remember the ARM7 patch but I’ve heard of it.
I do remember trimming games. The DS “Nitro” card held 32MB of storage, and every ROM was 32MB, even if the game didn’t take up that much space. A trimmer would remove the junk data. Some games, for anti-piracy purposes, would detect the junk data, so you couldn’t trim them. (If a game was less than 16MB, or 8MB, it would go on a 16MB or 8MB card, respectively; I don’t think there were 64MB cards, but there might have been.)