In no particular order...
Loved Russ's stuff in the Fiend Folio, but really started digging his work when I read the book Dicing With Dragons and the Fighting Fantasy game books.
If there's one artist who typifies old-school art to me, it's E.O. To this day, the AD&D Monster Cards are one of my favorite items in my RPG collection. And just in time for Christmas, the old-school game box adorned with Erol Otus art.
It wasn't Wormy that did it for me. (I actually felt like the Wormy strip, while interesting, was space in Dragon that could be used for other stuff more applicable to my gaming; no offense.) It was images like the mouth in the dungeon wall, or the cover of the PHB. Now, he's a 12th level Cabbie. (Does anyone know the prime requisites for that class?)
The reason I started playing V&V was because of all those great little ads in Dragon feature J.D.'s art and characters stats. It's an atrocity that TSR threw most of Dee's early D&D work in the trash. But if you're interested, Jeff has already more than doubled his kickstarter goal of $2,500 to recreate the Egyptian Art from Deities & Demigods. And he's got 9 days left to go!
Alan's work is an acquired taste. That's probably why he is one of the lesser known names, but I guarantee you remember his work. He's the guy behind the Crabmen, Dire Corby and Hook Horror illustrations from the Fiend Folio. (Please don't hold the actual concepts and content of the FF against Alan. He didn't create the monsters. He just drew them.)
Billman's another one you might not know as well. He was the guy behind most of the Lords of Creation art from Avalon Hill. Like James over at Grognardia, I regret never playing the game. Billman still does illustration, but in a more commercial/popular children's vein, but also has a full time in-house graphic design gig.
NOTE: I had a really hard time deciding which piece of art to showcase at the top of this page. Out of respect for all of the folks mentioned in this post, this is my first blog post (since I started the page) with no graphic at all.