Just what it says. This is a long and storied tradition, with most of the artists featured here responsible for more than a few record sleeves or CD liners. The following examples are just a brief sampling of ones I particularly like.
Rinaldo Ebasta - Bonnie E Clyde 45 (I have this framed on my wall and you don't; draw your own conclusions. The song itself is undoubtedly a contender for worst of all time, you can listen to it here.)
Kites - Hallucination Guillotine/Final Worship (the mixed-media packaging for Kites albums makes them almost like mini-CF zines)
RZA - Bobby Digital AND Kid Cudi - Man On The Moon (Sienkiewicz has done so much fantastic album art that I had to show two of his covers; his stylistic range on these things is also pretty impressive)
Joe Satriani - Surfing With The Alien (the kitsch factor on this bad boy iz off the charts)
Twin Sister tour 12"
Frank Zappa - Studio Tan
Big Brother & the Holding Company - Cheap Thrills (this one is probably the most ubiquitous of all, but hey, classic cover for a reason)
Wu Massacre: Meth, Ghost, & Rae
Sebadoh/Azalia Snail split 7" (apparently this is the only published art to come out of Columbia's tenure on Alan Moore's Big Numbers, a story told in full here.)
Black Pus - All Aboard The Magic Pus (Chippendale also does all the art for his other band, Lightning Bolt, but I figured people were less likely to have seen this one)
Feel free to drop links to images of your faves in comments -- I'll post 'em up if they aren't totally wack, and if I can find a good image, which is proving difficult. The first few are below, keep 'em coming.
Meat Loaf - Bat Out Of Hell
7 Year Bitch - Gato Negro
* Michael DeForge links me to this crazy comic-in-multiple covers by Mike Royer, which is more or less the greatest thing I've ever seen.
Gorillaz - G-Sides
The Supersuckers - The Smoke Of Hell