Girl Genius is excellent but not at all what I'd think of as manga. I've tried a dozen or so OEL Manga (Original English Language, a term I prefer to Amerimanga) but of all those I like Dramacon by svetlania best. I'm also enjoying what I've read of her Night School, available in two collections so far and monthly in Yen+ magazine.

She also has two webcomics: Chasing Rainbows and Night Silver, which I keep meaning to try.

Setona Mizushiro, who did After School Nightmare, also did the two volume X-Day, which I thought was good but not as good as the wonderful ASN.