I searched for a year for something free and I never found anything. Until about 2-3 months ago I used iMovie and took pics with my webcam which were saved automatically to my desktop. Then I imported them to iPhoto, then I would import media into iMovie and set the picture showing time o 0.03 seconds (the lowest poss just by clicking and dragging the lever). That way works OK as a free alternative. Now, however, I use iStopmotion Express. the cheapest version is $50 I think. It save a lot of time and is worth the money. Another option, which is cheaper, is FrameThief which is quite good too. I think you can buy that for $30. There is also iCanAnimate which is about $50 but this one runs differently (you may prefer it but I don't) All (in the end) produce animations. iMovie would still be needed to complete some final editing etc. All can have trial downloads so maybe try them out see what they are like.
Hope this helped.
EDIT:- Sorry I didn't realise how much I had bumped this and this may, by now, be completely useless.