A very
obscure and very good
manga that ran for awhile as a serial in
shonen jump and was eventually recompiled into 8 volumes. It's basically a more extreme version of
ranma 1/2, except it's been thought out a great deal more deeply. It's about a young boy named
Futaba who upon reaching puberty discovers he has inherited a very odd genetic condition which causes him to change genders whenever he becomes aroused. (His family had apparently been doing this his entire life; he just hadn't been paying much attention.) Futaba then has the bad luck to fall in love with a girl named Misaki.
The series is really funny, really weird, and gets very depressing toward the end. There is no available english version (Note: this is no longer accurate. See Andara's writeup below.); however, if you look, you can find transcripts of the entire series on the internet, at www.otakuworld.com. Go read them if you don't have anything better to do-- it's a vaguely kick-ass series even without pictures.
I could explain it/interpret it further, but i don't need to. The series explains itself. Go read it.