Gale Force Nine publishes official D&D reference cards. I have a set with magic items and several sets for various spell lists. I have two sets for monsters, one for Challenge 0-5 and one for Challenge 6-16. Unlike other sets, many monsters require a lot of information so most cards are double-width. As usual each card has a picture in front and statistics in back. The cards are sturdy and have glossy fronts and backs. Monsters with variants tend to have variant cards (i.e. there is a card for young white dragon).

I haven't used the monster cards but I'd imagine in an adventure I'd take out the cards for monsters that appear so I have them handy without having perhaps multiple monster manuals around. Maybe in a card folder (like what I see Magic the Gathering players use) so they're not all spread out or in a pile that can be knocked over. I was also thinking of using it in Pendragon as a random monster generator, picking a random card and then making up a corresponding monster.

Sadly, with COVID-19 and less face-to-face gaming a product like this seems less useful. I guess as a GM I could still have it around but if we're doing remote gaming then having a PDF with the select monsters may be more useful (especially since I use three monitors so have the extra screen space).

Still, I think a useful add-on to a D&D campaign.