Indeed, Yozakura Quartet is still going in japan, and the eighth volume was released in April.
(The american edition might be on hiatus though. The fifth vol was released september, sixth was meant to be released in May 2010 but never came out and isn't listed on Del Rey's website).
Gekka Kajin Lumen Lunae is on hiatus (or discontinued) but it definitely isn't complete, and wiki seems to confirm that.
The author's blog (scroll to 2008年07月08日) also mentions that she's working on another series (Strawberry Palace) before looking into doing a sequel for Dragon Knights or a continuation for GKLL (「ドラ騎士続編や月華佳人の続きを描く前に、別の、あまり長くない筈、の、連載始まります」)
Pretty sure Gamerz Heaven also qualifies. The 4th volume came out back in 2005 and from what I've heard v4 doesn't actually 'finish' (and baka-updates has it as discontinued).
Oh, and Marichen Chronicle was also put on hiatus so the author could start a new series instead.
I was under the impression that Aqua Knight is on hiatus until Battle Angel Alita: Last Order is done (Wikipedia says so). I don't see it in the club relations, though. And, if you'll forgive me:
IT'S A DOUBLE HIATUS ALL THE WAY ACROSS THE MANGA INDUSTRY! WHAT DOES IT MEAN? (Better than Clamp with its TRIPLE hiatuses, though)
I recommend that Buyuden be added to the list of "discontinued" manga.
I know it is listed as Finished on MAL, but I read on other anime/manga sites that it was canceled by the author. On the Anime News Network site, it was said that the reason behind this cancellation was poor sales. So it is safe to assume that he meant for Buyuden to continue past Chapter 134, but couldn't due to external factors.
I just finished reading the last chapter. To be fair, the ending of this discontinued manga isn't as abrupt as others - such as Zen Martial Arts Academy. But it was painfully clear that the characters haven't been developed to their fullest potential and there are other characters who haven't yet re-surfaced as expected.