Anyone knows of a few manga that are either experimental, innovative or simply unique?

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Totally second Plastic Girl. I think you'd do well to try out Furuya Usamaru's other works as well.

Discommunication Seireihen and Gogo Monster are quite unique.

Milk Closet appears that way too, going by the summary.

You could also try the manga in the 'unstereotypical' and 'surreal' categories.

Interesting titles so far, I'll give them a shot.

Oh, I forgot to mention this before, but I prefer looking for them on both shounen and seinen titles.

I hope this helps.

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See ya!

As popular as it is, Full Metal Alchemist is pretty unique. It's a shounen but it breaks a lot of the shounen cliches.

Top Shelf recently put out an anthology of experimental manga from AX magazine. The book is simply titled AX Volume 1, check it out at your local comic store! It's pretty awesome, I devoured the whole book in one sitting. Some of the stories are so far out, while some are so sweet that they hurt. Others are so disturbing/haunting that they take up residence in your brain for quite some time. As for scanlations, it's not usually my reading fare, but I have enjoyed Mysterious Girlfriend X, and Franken-Fran. Hope that helps!

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Too Weird to Live. Too Rare to Die. - HST.

AFAIK, Black and White is pretty unique in terms of art style and takes the concept of "brotherhood" to a different level than most series.

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"When we remember we are all mad, the mysteries disappear and life stands explained."
- Samuel Clemens/Mark Twain

Someone posted about Milk Closet before me - my sustained campaign of recommending it must be working!

For less obvious ones ... I dunno - I look at my list and most things there are pretty unique. Not sure if I should just rattle them off or if you're looking for something specific in the uniqueness (artsyness, or weirdness, or ... something). Can you clarify a bit?

I'd second all the previous recommendations of Highway-STAR as well as the works of Furuya Usamaru, and I'd especially recommend Furuya's first work, Palepoli. As previous posters have recommended a couple of his works, I'd also point out that everything by Matsumoto Taiyo is worth reading.

I'd also recommend the following (all of these have graphic content):

Nijigahara Holograph - This is as densely packed as manga gets; a thoroughly nonlinear narrative in which the artwork is as important to understanding the story as the written words.

Ranman - A collection of many, very short stories; featuring the surreal art of Shinkichi Kato.

Journey to the End of the World - Not completely scanlated (nor is it likely to be any time soon), but it is certainly different from anything else I've seen.

The World is Mine - Not completely scanlated (it is completely available in French if you can read that), as I'd ramble in describing this manga, I'll simply link flyingrobots' excellent review.

Ryuguden - A peculiar story which has pacing appropriately quick for its capricious nature.

If you can enjoy (very) graphic content and black (and crude) humor, I'd recommend some of the more experimental oneshots by Shintaro Kago; such as:

Abstraction
Multiplication
Blow-Up
The Memories of Others
Everything's Peaceful

Also, many of the oneshots scanlated by either Kotonoha or Mangascreener would fit this category.

There's another that I read just recently and would like to suggest; Bakune Young. Very...odd, and quite surreal - but really enjoyable.

Oh, and I think maybe Soil, Homunculus and Obrigado! would fit as well. I didn't fancy them much and gave up reading a long time ago - but they're certainly unique.