Yeesh, that opens up a *lot* of territory, I'll list some standouts, but really, the Ranma section of my FFnet faves list is pretty much a glossary of Ranma-chan crossovers that I've found and liked. I'll avoid posting fukufic because those are pretty much all on the blog:
These will also be at most single crosses.
But first, here's a pure R1/2 fic:
Her War by Materia Blade is in progress and it is definitively Ranma-chan. That said, it is utterly brutal. I personally can't make it through the story, though it is written extremely well. I just don't have the stomach for it, Lost Innocence is about as dark as I can stand to read, or Starting Over, and this is at least an order of magnitude above them on the dark-fic scale.
I think this fic is old enough that you might have seen it, but I don't know the original posting date, plus I think it is wonderful.
A New Awakening by Jerry Unipeg and Dani Yanega is fairy!Ranko and very well done.
Chaotic Rebirth by Trugeta is a good pregnant Ranma-chan/SM cross. I'd like to see it finished or continued though.
Daimakaicho Ranma by Tuisto is a good start to a Ranma-chan as a demoness fic set in the A!MG universe. Like most AMG crosses though, it stops right when it was starting to get going.
Hikari no Daija by Miriani is a great HP cross but unfortunately hasn't been updated in a very long time.
Neko by Angel Frog is another good HP cross.
I rather enjoyed
Saiyajinnyanicuan by Trugeta even though I normally hate DBZ crosses.
That's all for now, there are a bunch of others that are significantly more incomplete, are listed on the sites that were posted, or are fukufics that are probably recent enough to qualify, but this seems like enough links for now.
I suppose I'll post one more even though it isn't necessarily Ranma-chan, she features quite a bit:
The Wild Horse Thesis by Calamity-Queen is a good Eva cross.