hollywoodcamerawork.us:

It has always irritated us that there are no green screen and VFX plates around the internet you can just download and play with -- everything is owned by some production. So we decided to donate some of the interesting plates from Visual Effects For Directors to the community, so you can download them and practice keying, matchmoving, compositing, tracking, and much more!

These look to be great if you want to practice your keying. Sometimes its not a easy as it looks.