Shigeru Mizuki
A Japanese manga artist best known for his "Ge Ge Ge no Kitaro" comics, many of which can be downloaded from this site.
His strange monster imagery is mostly based on Yokai legends.
Be sure to browse through all 3 pages of the gallery of his artwork at this site.

Oreli's Hypnotic Monsters
One of the artists associated with the "Crazy Dolls" website, these works stood out the most to me as some of the most interesting. This giant beast with its "monster brains" exposed as a giant bag of pink balloons is very inventive. The website is very well designed with a wide assortment of animated menus directing you to drawings, paintings and the peculiar fabric monster sculptures.

Link found thanks to les frères Guedin

Crazy Dolls
This gallery/shop contains a selection of different artists who have made bizarre creature oriented dolls, mostly out of fabric materials.

Clive Barker
I've been a huge fan of Clive Barker since I was a child. Films like Nightbreed and Hellraiser introduced me from a very young age to imagery that is both wildly surreal and fantastic as well as being horrifying and grotesque. I've had the honor of meeting Clive on several occasions and each time was lucky enough to have him draw me a picture. Strangely, each drawing is of a strange mask on a stick. I'll post scans on here sometime later.

Apart from having wrote over a dozen incredible books and directed many great films, Clive is a very productive painter. He's made thousands of wild and bizarre paintings and drawings over the past several decades. His latest series of novels, "The Abarat" combines his fantastic writing with his paintings.

Revelations, which has become the official Clive Barker resource, has a great amount of interviews, artwork and other information on Clive. Their section devoted to his paintings and drawings has a lot of interesting photos, many of which showing Clive in his studio.

Heinrich Kley
Kley was a German caricaturist, editorial cartoonist and painter. Much of his imagery focused on fantastic scenes depicting humorous and often disturbing imagery, some of which I've posted below. I found the picture above of the giant catterpillar attacking a woman especially creepy.

Toy trains animated like snakes.

A man having his insides pulled open by a crowd of miniature people.

People climbing out of the artist's skull and absorbing into the drawing surface.

Daniel Johnston
"Daniel Johnston is a prolific American singer, songwriter and artist. Johnston suffers from bipolar disorder and often writes surreal songs about his hallucinations and mood swings." Quote taken from wikipedia.

There's a film being released about Johnston, you can find more info about that here.

Bill Dunlap
Lots of drawings of monsters and other odd things on display here.

Mark Hosford
Here are some really amazing drawings of very unusual environments. I was amazed by the grotesque and surreal nature of the works displayed here.

Check out this gallery of the work of Roland Topor I recently found thanks to a link through LeWUB.