This is a question for TTRPG GMs. What RPG books, supplements, or accessories do you find yourself using year after year? Which RPG products provide the biggest regular impact at your table?
A wet erase map probably. Though I don't know if that's an 'accessory' since it'd be like saying dice, or notebooks are accessories they are so central to the whole thing.
I think one thing I really like since about last year is 'origami' miniatures i.e. miniatures folded out of paper, there's just something that speaks to me when I put down a bunch of paper frogs or a paper dragon as a miniature. They are 'abstract' but honestly it's not like it needs to be super detailed. Plus I don't have to worry about it getting damaged or lost, plus I can give it to people, which if you are running a game for kids, you'll want to bring more than just one paper dragon miniature.
As for supplements I quite like the Book of Challenges from 3.5e and WallyDM's Journal of Puzzle Encounters both help me with just coming up with challenges and puzzles since I can page through it a bit then make a new puzzle/challenge.
Second this. probably the most used accessory i have. Others come and go, but that vinyl map i've had kicking around for 20+ years is still used almost every game.