Again, most of you have probably known this for months, but...

You can use the region filter to do cropping and animated 
cropping within RexEdit.

Just set the outside effect solid color and change it to 
whatever you like (black maybe). You can then animate your 
crop by turning on moving path and setting keyframes for 
the region. By using the PiP option you can then position 
your cropped image wherever you like. Put another one on 
the insert track and use luminance key to make a collage 
of images. Pre-render that and add some more. Go nuts.