Rozen Maiden is the best anime series ever made, even with the bullshit copout ending.
It's about a modern-day 9th grader, Jun, who ends up with a collection of Victorian-era German dolls that walk and talk and all have eccentric attitudes. The doll's real names are cool shit like Jade Stern and Kleine Beere but they just use Japanese names in the series because the Japanese are insane.
The dolls are all fighting to become the perfect little girl because their creator is an asshole.
Rozen Maiden isn't popular just because it's a well-written story, the dolls have endeared themselves to actual doll collectors as well, limited-edition versions were released that are as valuable as a small car now. Japan's Prime Minister is -- believe it or not -- a fan of the series.
Suiseiseki is arguably the most popular of the dolls, being nearly bipolar with shyness and downright meanness. She also ends everything she says with "desu" (lit., "it is"). During the entire series, the total count of "desu" is 527. Many people on the internet enjoy this so much that the Church of Scientology, in early 2008, was left scratching their head when page after page of "desu" was sent to their fax machines. They probably think Suiseiseki is Marcabian.
Suigintou is probably the second most popular doll. She begins as a villain, but is proven in the OVA series (which is set between the first and second season, more on this below) to be pretty justified in her psycopathic rage. "Father", once again being the dick he is, stitched inverted crosses on her dress in order to scare the Christian Right whenever she decided to go into US Politics. By the way, "father" is identified as the Count St. Germaine, an actual historical figure with a real fruity occultic legend behind him.
Whenever a comic makes the transition to animation, you can always rest assured that the animators are going to screw it up somehow.
As of July 2009, chapters of the comic are still being released by the creative team. Whether or not this will be complimented with animation is unknown, but the ending of the series (as bad as it was) leaves itself open to the possibility.