Fairest and Ella Enchanted by Gail Carson Levine - Yes, there was a movie made out of one of these. Fairest is the one I found first, and my favourite of the two books. The two are not technically a series, but they take place in the same universe, and some character do cross over. Both books are fairytale retellings, with engaging characters and roller-coaster plots, set in an interesting universe. (I have also read her book "Ever", which was quite good as well.)
The Life and Times by Jewels5 - Okay, so, technically, its a fanfiction. But it is more entertaining than the actual books, so as far as I am concerned, it still counts. Incomplete, as of then this post was made, but progressing at a relatively steady pace. I've been reading it for almost three years now and I check every month for a update. If that doesn't say something about the story then
I would have added Good Omens by Terry Pratchett and Neil Gaiman to the list, but I read it because a couple of the characters appeared in a webcomic I enjoy. I'm not sure if that technically counts as a recommendation, but all the same. Good Omens is quite possibly the best book of all time, by the way. The Gem Series (Ruby Red, Sapphire Blue, Emerald Green) by Kerstin Gier almost made this list as well, but I am reserving judgment until I have finished reading all of the books. So far I have only read the first book, and that was a year or so ago. When I finished I was chomping at the bit for the next instalment, but it hadn't been translated yet, at that time. So I must have really enjoyed it, even if I seem to have forgotten many of the details now.