eharlequin.com
They have various "clubs" where you'd receive a book or two every month in one of their series lines. But you can get 2 books from each line completely free; you just have to remember to write "cancel" on the bill and send it back right away.
BTW, they aren't just the typical Harlequin romances. Some lines are geared toward suspense, mystery, thriller, or the paranormal.
It appears you can request freebies from two series a month (judging from the error messages I got when I tried to order more). So that's 4 free books a month, til you run out of series to try.
I know they have a 2-book freebie offer on Start Sampling quite often, but I never knew til last night you could get even more. So I thought it was worth a post.