Here are some of my personal favorite monster/gimmick girl romances:
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Tasogare Otome X Amnesia - Soft, stationary ghost. One of my absolute favorites.
Voynich Hotel - Brown maid/witch. It's Dowman Sayman, so you know it's good.
Duke of Death and his Black Maid - Goth maid/witch and her Tim Burton-esque love interest.
Jitsu wa Watashi wa - Dumpire et al. Also, a good "harem" with progression.
Pseudo Harem - One girl-harem. Also has romantic progression.
Karin / Chibi Vampire - Nosebleed vampire. Used to be decently popular at one point in time, then fell off the face of the Earth.
Oh my sweet Alien - Tentacle, pupil-less alien wife. Very cute, and has Miyata Kouji's cool art.
Mousou Telepathy - Big tiddy telepath. This one's gonna be ending soon, but NOBEL's other webcomic is also good.
Pandemonium - Bird-lizard-fox furry. Short and sweet, so Western-looking it reads left to right.
Shuukan Shounen Girl - Romantic oneshots, each featuring a different unique "gimmick girl".
Oddman 11 - Yuri Scott Pilgrim. Another smart and slutty Dowman Sayman manga.
Depression with Succubus - Dangerously flat succubus. These "God, I wish I had a good/pure succubus mommy to take care of me" manga are pretty cheap and common, but I particularly like this 4koma.
Midori no Hibi - Tiny right hand girl. Like Karin, also used to be pretty popular, only to be quickly (and undeservedly) forgotten. The anime cut a lot from the manga, so give it a shot if you haven't already.
Ano Hito no i ni wa boku ga tarinai - Whale gf. Extremely underrated, but has been slowly gaining popularity recently.
Jigokuren - Satanic torturer demon girl. Rough around the edges, but genuinely fun gore-comedy manga.[/ul]