This one is for the old timers...
What are some of your favorite first- or third-party accessories that you bought, or saw for sale, to help people play D&D and other RPGs? What were some of the weirdest you remember seeing? What parts of the game did they shore up? Do you have any stories about the companies who made them?
I loved my 1E DM screen to death - that thing was so helpful back in the day; and I loved that it had the weapon chart facing the players.
Gamescience dice changed the way I looked at dice, and which ones I bought thereafter (and had Lou's speech just about memorized for all the times I bought Coastcon dice from him

).
On the computer side, I have to give a big shout out to Profantasy Campaign Cartographer. I used it for many years to design campaign maps and dungeon maps that could be rescaled as needed - I even printed a few out as battlemats. It was a nightmare to learn, though.
The switch from metal minis to prepainted plastics had a huge impact for me; I've still got metal minis back from the day that are still just primed white and kept in padded drawers of a bit box bin (y'know, the type most folks keep screws, nails and whatnot in).
Likewise, going from battlemats & dwarven forge pieces to the die-cut dungeon tiles made on-the-go DMing so much easier for me. Didn't take up as much space and as long as I planned ahead, looked a lot better than a scribbled-on battlemat (especially the three I ruined with red wet-dry markers*).
Come 5E, buying a set of condition tokens was so helpful - I can just hand out a poker-sized token that has the details on it so players are reminded of any conditions that effect them - no more forgetting they've been
Blessed, or if they've received a
bardic inspiration.
The one item I remember seeing from way back when that wrinkled my nose was the (TSR?) Combat Wheel. It was just too nerdy - in a slide rule sort of way - that I just wanted to stay far, far away from it. That and Dragonstrike and it's cringeworthy VHS tape.
* Though years later, when Mr. Clean magic erasers came into being, I was finally able to clean those mats to make them no longer be an eyesore - though I did end up needing to re-ink some of the lines.