Compositing Software

These packages are the most popular and flexible ones for combining images:

Jahshaka – free, open-source video editor and compositor.  This project seems to have risen from the dead.

Blender is technically a 3D program, but it has powerful effects and compositing abilities as well.

Natron not only promises the usual compositing tools, it seems to work with plugins for well-established compositors like After Effects.  Free and Open Source.

• It’s Windows-only, but Wax offers some of the same capabilities.  Your mileage may vary.

Adobe After Effects offers a 30-day trial at Adobe.com.

Nuke – The former in-house compositing program at Digital Domain is now a major industry tool.  There is a 15-day trial available here.

Online fora are good sources of information about how specific products work and for user-written extras that might be useful.

Lots of video editing packages offer compositing options as well.