Doujin? Yes. By different artists, meaning some of them look really cute
(but relatively clean), some are a bit more ecchi, and some just have rotten art. Think of them as similar to OS-Tans, only for Kamen Rider. Essentially it's a parody of the characters; the only people I've found who were offended by them were guys who take their Tokusatsu way too seriously.
Best representative in art style terms would be Kazuu:
http://www.geocities.jp/kazuu_hp/
Faiz-Tan:
Blade-Tan:
Kabuto-Tan:
In fact, it was popular enough to have some merchandise. Meet TheBee Zec-Tan:
http://www.geocities.jp/kazuu_hp/tokusetu2.htm