Gwyllion
Here are a couple of banners for my own site. An "R" rated one and... er, one that's less "R" rated. Hey, if you can't handle the content of my banners, then you probably shouldn't be linking to me anyhow.
Adam Hughes - DeviantArt
http://adamhughes.deviantart.com
Another one of my very favorite comic artists. The Wizard of Ah's. If you read comics, then you have probably seen his work. Yeah, he's that good.
Aimee Major's Links
http://aimeemajor.com/wp/links/art-links/artists/
They used to have a much more comprehensive list of artist links, but still worth looking at.
Al Rio
Al Rio passed away in 2012. The site remains.
DeviantArt
If you don't know about DeviantArt, well, all I can say is that you've been missing out, big time. Comic book artists, illustrators, photographers galore. A lot is garbage, but get a good collection of artists to "watch" and you'll have a near constant stream of fantastic art coming at you 24/7. While I don't personally have anything there (1 piece), it's a resource I check many times a day. If you need a place to start, try my own favorites. http://mbg315.deviantart.com/
Devilkitten/Fainted Sun
http://homepage3.nifty.com/devilkitten/
One of the artists I check frequently. Very talented.
E-Hentai Galleries
http://g.e-hentai.org/tag/tentacles
This is pretty much my "goto" search site for anything hentai/manga/tentacle related. This place has almost everything - artist, manga, doujinshi, you name it. Even my own works are there. You can customize your searches very nicely, and most of the works even have torrent links. Great resource.
e-jin
http://e-jins.sakura.ne.jp/blog/
Probably my favorite tentacle artist. Very gifted, both in drawing skill as well as imagination. I love this guy's sketches. Great imagination.
Eerie Cuties
More comic goodness from the same author as Ménage à 3 (Gisèle "Giz" Lagacé). Not quite a risque as the former, but still a fun morning read, with Vampires, possessed dolls, and succubi, oh my!
Eldrik Aethervial's Oddities
http://eldrikaethervial.tumblr.com
Some of you might remember this artist as bobbydando. Well, their new tumbler site is chock full of amazing tenti-art and comics. Definitely one to watch.
Faith
http://www.wasurena.sakura.ne.jp/~faith/
Again, a Japanese site, so once again I have no idea if this artist is a he or she. Awesome tentacle works (has several pay collections), and their detail is amazing. Watch out for his "gore" works. They are pretty damn explicit. I'm not into the ripping and tearing, but you have to appreciate his skill. The bloody stuff is in it's own section (whew!) so you don't ruin your breakfast.
Frank Cho
http://www.libertymeadows.com/
As a nationally syndicated cartoonist, Frank Cho is very much the pro. You might know him from Liberty Meadows, or possibly a whole slew of fantastic Cavewoman art. One of the things I love (and admire) about Cho is his ability to draw the female figure with weight. His figures have mass that set's them on the ground, not just that nearly-levitating quality that most comic artists draw (esp. Al Rio). Take a look at Brandy from Liberty Meadows. She's any male's wet dream, and yet Cho doesn't have to resort to a Barbie-sized waist and high heals to make her that way. And check out those ankles! Now that's realism baby.
Ganassa
http://ganassa-artwork.blogspot.com
This artists does a lot of game girls, and has a very recognizable style that is quite good.
Hyung-tae Kim
This artist is a virtual god in the gaming industry and his works are incredibly humbling.Update 3/14/07 - Looks like the gallery is offline (bummer).Update 4/30/08 - Looks like it's back again. Yeah!
Junny
Another Japanese artist, Junny is both a great sketch artist, and a great colorist with their own distinct style. I love the loose feel of their art and the mastery of light. Worth watching for updates, though I think it's been a while...
Ménage à 3
One of the web comics I follow. I have always been in awe of artists who can produce such obviously fantastic quality with prolific regularity. This comic is Archie, but with all the sexy situations you imagined Veronica in as a teen.
Oglaf Web Comic
More web comic fare. Definitely adult in nature. Great art and the humor is completely original. Updates on Sunday!
Poulpi
http://pmchan.free.fr/index.htm
French artist with a flair for detail and awesome coloring. Very cool stuff. Hasn't updated in a while though.
Spying with Lana
http://www.amazingartbros.com/Lana
Just a fun and sexy web comic detailing the adventures of a very promiscuous spy.
Tentaclerape.net
Pretty much what the name implies, though they cover more than just tentacles. This site is pretty damn up to date with the latest art within this rather specific genre, and is a regular for me.
The Cummoner – Web Comic
A very well drawn web comic. Very sexy. Yum.