Doujin is a term used for self-published works in this case manga.
Most of them will feature characters from popular series of anime, manga and games.
But there are also great original works.

Most Popular Series: Naruto, One Piece, Bleach, Dragon Ball

Zoro x Nami Doujin (One Piece)

Zoro x Nami Hentai

Mangaka: Kurione-sha
Genre: big breasts, comedy, zoro x nami, vanilla
Size: 12MB 18pages

Momonosuke Fucks Nami and Robin

Momonosuke x Nami

Mangaka: Isao (Majimeya)
Genre: momo x nami, momo x robin, threesome sex, femdom, shotacon
Size: 14MB 26pages

Perona x Kumashi Doujin (Ghost Princess)

Perona x Kumashi Sex

One Piece – Ghost Princess – Perona x Kumashi (English)
Mangaka: Chikasato Michiru
Genre: anal sex, furry, big penis, inflation, deformation, vomit
Size: 12MB 19pages

Nami Impregnation (One Piece)

Nami Impregnation

One Piece – Nami Impregnation Doujin (English) [Uncensored]
Mangaka: Dashigara 100% (Minpei Ichigo)
Genre: dilf, prostitution, pasties, creampie, nami pregnant
Size: 14MB 26pages

Sanji Fucks Robin Hentai Doujin

Sanji Fucks Robin Hentai Doujin

One Piece – Sanji Fucks Robin and Nami (English)
Mangaka: Konohana
Genre: sanji x robin, threesome sex, dream fantasy, x-ray
Size: 17MB 25pages

Nami x Robin Futanari Pirates

Nami x Robin Futanari Pirates

Nami x Robin Futanari Pirates (English)
Mangaka: Musashino Sekai
Genre: big tits, futanari, dickgirl x dickgirl, femdom, mind break, x-ray
Size: 16MB 23pages

Usopp x Nami Sex Doujin (One Piece)

Usopp x Nami

One Piece – Eromance Dawn (English) [Uncensored]
Mangaka: Bobobo
Genre: Usopp x Nami, Prostitution, Creampie, Impregnation
Size: 12MB 25pages

Tsunade-sama Got Pregnant

Tsunade-sama got pregnant

Tsunade-sama Got Pregnant (English)
Mangaka: Daigo
Genre: Big Tits, Tsunade x Shinobi, Virgin, X-Ray
Size: 11MB 25pages

Nami Gets Raped by Jack (Milking Tits)

Nami Gets Raped Milking Tits

One Piece – P.O.M Another Episode J.A.C.K (English)
Mangaka: Bonten
Genre: Huge Tits, Nami x Jack, Drugs, Lactation, Mind Broken, Rape
Size: 63MB / 16MB 26pages

Boa Hancock x Kaido (One Piece)

Boa Hancock x Kaido

One Piece – P.O.M Amazon Lily Last Day (English)
Mangaka: Bonten
Genre: Big Ass, Huge Tits, Boa x Luffy, Boa x Kaido, Mind Control, Muscle
Size: 67MB / 13MB 26pages