Well, theres no way you can get a high quality slr for under 500 unless its used and/or without a lens. I use a mac and use A canon powershot sx110 is. I have had some major problems with it. I would go for a firewire camcorder, not sure what model or brand, sorry. Well, the last option is to use a webcam like the qcp9000 with virtual machine software like vmware fusion to run windows capture software side by side in os x. Sorry I couldn't help.

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Actually the Canon Rebel XS is a really good camera for stop-motion and it's only about $500-600, granted it's not UNDER 500 but it's around, so it's fairly cheap for a DSLR. And there isn't really such a thing as a 'low quality' DSLR, there DSLRs there built to be the best quality. I actually shot on a Canon 7D not that long ago for both a still image project (I took a scene from a movie and used the audio and still images to play out the scene) and a stop-motion animation class I took. But using the 7D was overkill for stop-motion. You can easily get away with something cheap like a Rebel XS or a T1i/T2i.

I don't actually know how advanced you are with stop-motion, but a DSLR might be a bit too much. You can still get really good picture quality from a good Canon point and shoot camera.