If you take a look at my reading list in the past year, you'll notice that a large percentage of the books I've read are from the Aubrey/Maturin series, which follow the adventures of a British naval captain and his surgeon during the Napoleonic wars.
I've read eight books in the series thus far (there's twenty-one total) and have raved about some (H.M.S. Surprise and Desolation Island, in particular) and detested others (like The Mauritius Command), but eight novels is a lot. I've never gotten so deep in a series before, and I've been wanting to read other things too just for some variety - yet I can't seem to stop, and I think I finally understand why.
The Aubrey/Maturin books almost always finish in the middle of major action; usually a battle. The endings typically occur moments after a victor has been declared.
It's not a cliff-hanger, because there's usually no indication of what will happen in the next book. There's no unresolved conflict, mystery, or questions (besides some long-term, more minor plot points). The books just tend to end on the climax of the story without any kind of denouement. It's very addictive, because you finish the book on such a high note that you can't help but want to read more.
So, this is just a tip or an idea for you writers out there. If you're planning a series, maybe consider finishing your books in the middle of the action/climax. You don't have to leave questions unanswered, necessarily (though you can, that's a sure fire way to bug your reader and get them to purchase/borrow the next book), but maybe consider getting rid of the boring denouement altogether since it can leave the reader on a down note, which doesn't immediately motivate them to pick up the next book in the series. I have to say, it's really an effective method to finish the book in medias res (i.e., in the middle of the narrative), rather than using a boring, if tidier ending. It's the main reason I keep slipping the next Aubrey/Maturin novel into my library haul.