Pixiv

Pixiv is a Japanese online community for artists, not unlike the Western world's Deviant ART. Launched in September 2007. Pixiv aims to provide a place for artists to exhibit their illustrations and get feedback via a rating system and user comments. Works are organized in an extensive tag structure which forms the backbone of the website. Registration required to view the majority of contents.

In case that doesn't sound like anything special to you, consider that the majority of Anime and Manga in the 'boorus and the 'chans are taken from Pixiv, most of the time without permission from the artists.

If you're using Firefox with the Greasemonkey extension, the Pixiv Translation Plus script may be helpful.

Compare Nico Nico Douga (Japanese YouTube).

Oh and by the way, if you find relevant pictures, consider visiting the Image Links Wiki.

All links below should be assumed NSFW, just to be safe.

Random works tags, with characters where available:

 * A Certain Magical Index: Here.
 * A Certain Scientific Railgun: Here.
 * AIR: Here.
 * Angel Beats: Here.
 * Aria: Here.
 * Arcana Heart: Here.
 * Armored Core: Here.
 * Axis Powers Hetalia: Here. (Nearly 350,000 images.)
 * Azumanga Daioh: Here.
 * Bakuman｡: Here.
 * Black Rock Shooter: Here.
 * Blaz Blue: Here.
 * ''Bleach: Here.
 * Blue Exorcist: Here.
 * Breath of Fire: Here.
 * 2 is here and here.
 * 3 is here. and here.
 * 4 is here and here.
 * 5 is here. "Dra Kwo" or something like that stands for "Dragon Quarter".
 * Cardcaptor Sakura: Here.
 * Cave Story: Here.
 * Chrono Trigger: Here.
 * Code Geass: Here.
 * Danball Senki: Here.
 * Dennou Coil: Here.
 * Digimon: Here.
 * Disgaea: Here.
 * Dissidia Final Fantasy: Here.
 * Doraemon: Here.
 * Dragon Ball: Here.
 * Dragon Quest: Here.
 * Durarara: Here.
 * El Shaddai: Ascension of the Metatron: Here. (Yea, it's ridiculously popular there. Perhaps something about Clothing Damage that becomes Shirtless Scene...)
 * Fallout series: Here.
 * Final Fantasy: here.
 * Fire Emblem: Here.
 * Fullmetal Alchemist: Here.
 * Gintama: Here.
 * God of War: Here.
 * Guilty Gear: Here.
 * Gundam: Here.
 * Hamtaro: Here.
 * Happy Tree Friends: Here. Didn't expect that, huh? Also, for whatever reason, the Anthropomorphic Personification of those... poor critters are common, about ten thousands illustrations total.
 * Harry Potter: Here.
 * Hidamari Sketch: Here.
 * Higurashi no Naku Koro ni: Here.
 * Homestuck: Here　and here.
 * How to Train Your Dragon: Here.
 * Hunter X Hunter: Here.
 * The Idolmaster: Here.
 * Inazuma Eleven: Here. (Over 150,000 images.) And some more here.
 * Inazuma Eleven GO has a separate tag here with over 50,000 images. The tag for Chrono Stone can be found here
 * Kirino Ranmaru: Here.
 * Kariya Masaki: Here.
 * Matsukaze Tenma: Here.
 * And there's even yet another separate tag for "Inazuma Eleven [Yaoi ]".
 * Fubuki Shirou: Here and here.
 * Kiyama Hiroto: Here.
 * Kazemaru Ichirouta: Here.
 * Endou Mamoru: Here.
 * Sakuma Jirou: Here.
 * Kidou Yuuto: Here.
 * Fudou Akio: Here.
 * Irisu Syndrome: Here. There are high risks of spoilers if you have not played the game yet.
 * Kamen Rider: Here.
 * Kannagi: Here.
 * Katekyo Hitman Reborn: Here.
 * K-On!: Here.
 * Kirby: Here.
 * Legend of Galactic Heroes: Here
 * The Legend of Zelda: Here.
 * Little Busters: Here.
 * Love Plus: Here.
 * Lucky Star: Here.
 * Mabinogi Fantasy Life: Here.
 * Macross: Here.
 * Magical Girl Lyrical Nanoha: Here.
 * Magic: The Gathering: here.
 * Mariasama ga Miteru: Here.
 * Metroid: Here.
 * Monster Hunter: Here.
 * Nabari no Ou: Here.
 * Neon Genesis Evangelion: Here.
 * Nichijou: Here.
 * Ojamajo Doremi: Here.
 * Okami: Here.
 * One Piece: Here.
 * Pandora Hearts: Here.
 * Pangya: Here.
 * Pani Poni Dash!: Here.
 * Panty and Stocking With Garterbelt: Here.
 * Peanuts: Here.
 * Persona 3: Here and the PSP version here.
 * Persona 4: Here.
 * Pokémon: Here. (About 250,000 images.)
 * Pikachu has his own tag here.
 * Pop N Music: Here and here.
 * The Prince of Tennis: Here.
 * Portal/Portal 2: Here, Here.
 * Pretty Cure: Here.
 * Suite Pretty Cure: Here.
 * Heartcatch Pretty Cure: Here.
 * Fresh Pretty Cure: Here.
 * Yes! Pretty Cure 5: Here.
 * Puella Magi Madoka Magica: Here.
 * Queen's Blade: Here.
 * Ragnarok Online: Here.
 * Rockman/Mega Man: Here.
 * Rozen Maiden: Here.
 * Saki: Here.
 * Sakura Wars: Here.
 * Sayonara, Zetsubou-sensei: Here.
 * Sailor Moon: Here.
 * Secret of Mana: here for the series in general.
 * Secret of Mana the game here.
 * Seiken Densetsu 3 here.
 * Legend of Mana here.
 * Sengoku Basara: Here.
 * Sherlock Holmes: Here. With abundant amount of Ho Yay.
 * Sonic the Hedgehog: Here.
 * South Park: Here. Kenny and co are suddenly Bishounen.
 * Strike Witches: Here.
 * Studio Ghibli works: Here.
 * Summer Wars: Here.
 * Super Mario Bros.: Here.
 * Super Smash Bros.: Here.
 * Tales (series): Here.
 * Tales of Vesperia: Here.
 * Tales of Symphonia: Here.
 * Tantei Opera Milky Holmes: Here.
 * Teenage Mutant Ninja Turtles: Here. Didn't expect that, huh?
 * Tengen Toppa Gurren Lagann: Here.
 * The Melancholy of Haruhi Suzumiya: Here.
 * Tiger and Bunny: Here.
 * Touhou: Here. (Over 1,100,000 images, the single most popular tag on Pixiv.)
 * Cirno: Here. (Waaay over 9,999 images. The most popular, in fact.)
 * Kirisame Marisa: Here.
 * Hakurei Reimu: Here.
 * Remilia Scarlet: Here and here.
 * Konpaku Youmu: Here.
 * Iyazoi Sakuya: Here.
 * Flandre Scarlet: Here and here.
 * Yakumo Yukari: Here.
 * Yakumo Ran: Here.
 * Shameimaru Aya: Here.
 * Alice Margatroid: Here
 * Wriggle Nightbug: Here.
 * Reisen Udongein Inaba: Here.
 * Toward the Terra: Here
 * Transformers: Here.
 * Tsukihime: Here.
 * Unlimited Saga: here.
 * Usavich: Here.
 * Vision of Escaflowne: here and here.
 * Vocaloid: Here. (Over 260,000 images.)
 * Hatsune Miku: Here.
 * Kagamine Rin: Here.
 * Kagamine Len: Here.
 * Mandatory Warhammer 40000 example: Here.
 * Yu-Gi-Oh!: Here.
 * Yume Nikki: Here.

Tags dedicated to particular tropes:
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 * Animal Ears: Here.
 * Cross Dresser/"Trap": Here. Or Here.
 * Cute Little Fangs: Here.
 * Digital Avatar: The majority of users (who aren't Lurkers) have one themselves, but a phenomenon in Pixiv is creation of Digital Avatar of internet celebrities.
 * Nico Nico Douga's "Utatte mita!" Channel: Here.
 * Fantasy: Here.
 * Furry Comic: Either "BEAST" tag (Here) or "Therianthrope" tag (Here).
 * On that note, the "Animal" tag (Here) excludes Furries. Generally.
 * Glasses Girl: Here.
 * Gorn: Tagged with "R-18G", as opposed to "R-18" which is due to sex: Here. You've been warned.
 * Halloween Cosplay: Here.
 * High School AU: Here.
 * Hime Cut: Here.
 * Humongous Mecha: Here.
 * Kawaisa: Here.
 * Kimono: Here.
 * Kitsune:
 * Pixiv Fox Land: Here.
 * Maid: Here.
 * Memetic Mutation:
 * Caramelldansen ("Uma Uma"): Here.
 * Image Macros ("Image Response"): Here.
 * Take it easy!: Here.
 * New Year Has Come: Well, New Year Cards: Here.
 * Original Character: Here. (Sturgeon's Law applies.)
 * "Original Character, Don't Steal": Defied with the tag "It's OK if you want to draw him/her/it yourself": Here. (Sturgeon's Law applies.)
 * Panty Shot: Here.
 * Parody: Here.
 * Rule 63: Here.
 * Ryona, which is extreme violence done to women: Here. You've been warned.
 * Sailor Fuku: Here. (This tag is understandably one of the most popular.)
 * Scenery Porn: The tags means "I wanna go there!": Here.
 * For the Scenery that may or may not being Porn, Here and Here.
 * Super-Deformed: Here.
 * Take That Scrappy: Just like Danbooru's Everybody Hates Kyubey, Pixiv has We Wanna Beat Up QB.
 * This Is Wrong on So Many Levels: The tag means "How come it become like this": Here.
 * Tokusatsu: Here.
 * Tomboy ("boyish"): Here.
 * World Building: Aside from those created by users for existing works, there are also some World Building events organized by Pixiv itself. Well, it's really an art competition of creating illustrations for a Fantasy world described by Pixiv. A steady source of Crowning Moments of Awesome.
 * Pixiv Fantasia: Here.
 * Pixiv Fantasia II: Here.
 * Pixiv Fantasia III: Here.
 * Pixiv Fantasia IV: Here.
 * Pixiv Fantasia V: Here.
 * Yaoi: Here.
 * Yonkoma, or 4-panel comic: Here. (More like media format, but whatever.)
 * Yuri: Here.