Until comparatively recently, I'd never met anyone else who'd seen, let alone owned the Advanced Dungeons & Dragons Monster Cards. Released in four sets of 20 cards in 1982, they were apparently not widely released, thereby explaining their relative uncommonness. I owned all four sets, because I owned everything TSR released for D&D back then and because I genuinely believed I'd get a lot of use out of these cards -- except that I didn't.
Each monster card is roughly the same size as a large index card. On the front side is a full-color illustration of the monster in question and on the reverse are abbreviated Monster Manual stats and some descriptive text. Each set included 17 standard AD&D monsters and three new ones. Several monsters made their debut in these cards, but I can recall only one -- the thri-kreen -- that stands out as having become a new classic, unless you count obliviax moss or the galeb duhr as "new classics." And of course the thri-kreen itself is, in one of those ironies that often afflicts overly litigious corporations, a knock-off of Arduin's phraints. The real interest of the cards, though, is the new art in a wide variety of styles, from the phantasmagorical Erol Otus to the comic book stylings of Jeff Dee. Some of the illustrations are better than others, of course -- I particularly dislike Jim Roslof's kobolds, for example -- but I have long felt that one of the great strengths of the Golden Age was its esthetic diversity, which stands in contrast to later ages, which seem to have trade that diversity for better overall art direction.
I presume the intention behind the cards was that the referee could use them in play as a handy reference without having to consult the Monster Manual for stats. That's certainly how I'd assumed I'd use them. The problem is that, even in 1982, AD&D monsters were still simple enough mechanically that there was little to no need for such reference tools. Most monsters could have their stats written on a single line of two-column text and the rest required two at most. Likewise, monster abilities were simple enough that, so long as you'd used a monster before, you could pretty easily remember how they worked. It wasn't a matter of "rules mastery" or having a photographic memory so much as the fact that AD&D, at its root, was still a simple game. You really could keep it all in your head without the need for constant page flipping and chart scanning.
This meant the monster cards were, ultimately, attractive but impractical curiosities. I suspect that TSR hoped more gamers would see them as essential than did so, which might explain why there were no more sets after the initial four releases. The new monsters were all eventually published elsewhere (in the Monster Manual II), but, with the exception of the thri-kreen, none has had a lasting influence over the subsequent development of D&D -- little wonder then that so few people remember these products.