FanficRecs [taken from main page]
This started out as a crack fic and then rapidly developed an actual plot, complete with both Heartwarming and Awesome Moments. Harry meets and makes friends with Ranma (watching them try to play the "who had the worst childhood/mentor" game with each other was worth the price of admission all by itself), partners up with the ghost of Usagi (whose body is Harry's Jusenkyo-curse form), makes friends with the Chinese Amazons (but avoids the Kiss of Marriage thanks to Ranma's timely warnings), and develops an entire support structure in the form of the Senshi (and Ranma and Shampoo), to the point where by the time he has to return to Hogwarts for his sixth year, Dumbledore and Voldemort both have no idea what's about to hit them. In the meanwhile, Harry and Usagi have fallen in love... which is a bit of an obstacle, given that only one of them is alive, and the other one can only temporarily possess her body when her beloved is morphed into it... fortunately, she gets better. Tackles and answers the tough questions, such as "What happens when you cast a Patronus Charm through the Moon Healing Scepter", "How quickly can Snape's lack of tact earn him a nine-on-one gang beatdown by an enraged mob of magical girls", and "Just what the heck was Dumbledore smoking, to make some of the decisions he did?" Really, it's far, far better than it sounds, and I highly recommend. Also contains perhaps the only non-annoying appearance of Chibi-Usa I can remember seeing anywhere.
FanficRecs Seconded
Definitely seconded. I can't add much that isn't well covered by those above me, but very early into the fic was the line that caught me and hooked me on the whole story: Ranma explaining to Harry some of the nuances of having a Jusenkyo Curse: "Let me tell you, water will actually travel uphill to get to you now. You remember logic? Forget it. Doesn't exist for you anymore."
FanficRecs [taken from main page]
Comartemis}}: Wow. This is probably one of the best examples of Growing the Beard I've ever seen in a fanfic before. The first few chapters are really rough, characterization is spotty at best, several characters are Flanderized to hell and back, Dumbledore, Ron, Ginny, and Molly all get their usual Ron the Death Eater treatment, and while mpreg really doesn't ever show up outside of conversation, the way the subject becomes plot-relevant is absolutely absurd on the face of it. At some point, however — I think it's right around the same point where the fic turns mildly serious and stops being a Crack Fic — everything shifts in a generally positive direction. Mamoru is still a complete manwhore, sure, but he doesn't show up more than once or twice before he falls down a plot hole and isn't heard from again. Ginny never really gets better and Molly's attitudes are only weakly justified, but Hermione recovers from her Flanderization, Ron seems to get over his exaggerated jealousy of Harry, and the much-hated Manipulative!Dumbledore actually gets deconstructed later in TGWL. And while Chibi-Usa has never really irritated me like she has other people, I have to agree with Chuckg that she's very well-used here, possibly because her interactions with Harry are infinitely more adorable than her canon relationship with Usagi. It may not hold a candle to some of the other examples on this page but this is definitely one of the best HP/SM crossovers I've ever read.