Hmm, nobody mentioned Princess Princess yet...
Well, the art is great, the characters are awesome, and the plot is enough to keep you reading; I totally recommend it! ^o^

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Never Give Up is another one, but it's another girl-disguised-as-boy, not the boy-dressed-as-girl that the original poster was looking for.

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and My Barbaric Girlfriend

I super second My Barbaric Girlfriend. It's hilarriouuuus. But it's very different from Hourou Musuko so I'm not sure it's quite what you're looking for >.<

Oh yeah, there's also Okama Report. It's kinda old so once you get used to the art, it's really entertaining (comedy).

Oh and Please Save My Earth, had a bit of crossdressing (in a way, well a girl reincarnates into a guys' body). That was a really good series, although the gender bender wasn't the main plot. This is also a pretty old series, so one may not like the art too much either.

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Hey thanks everyone! Some nice suggestions in there. A lot of these I've heard of before, but forgot about. There were also some ones I hadn't heard about before. Thanks again everyone! And if you think of anymore please post.

Removed.

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This one's cute.

Penguin Revolution

It's more into showbiz stuff and there's more girl-to-guy than guy-to-girl, but all in all it's really cute and sweet. Much recommended if you like cute stuff.

EDIT:

ack. It was already posted, I didn't notice. Sorry. 😃

Princess Prince is really good as is Kill Me, Kiss Me. Those both have guys dressing as girls.

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k

Three other guy into girl mangas are:

Gacha Gacha: Secret (A.K.A. Gacha Gacha: The Next Revolution)
Kedamono Damono
Metamo Kiss

I'll post more as I find them.

Family Compo - 's missing here...

SeeYa

Gorath

W Juliet is one of the best boy-into-girl around >_>

What about Nosatsu Junkie? It's actually pretty good o_o