Natron

Node-based compositing is popular because it allows for major image manipulation with no enforced chain of dependency. You can rotoscope and resize and re-colour an image, and then remove any of those nodes without affecting the effect of the others. It's a powerful system used in most professional FX packages like Blender.

Natron benefits from several open technologies and open standards, not the least of which is OpenFX. OpenFX-compliant effects, both open and closed-source, from several companies and open projects are compatible with Natron.

Natron uses several professional, industry-standard technologies on its backend, including OpenColorIO and OpenCV. It fits right in to established industry pipelines.

The usual warning about open source projects not being discount versions of famous closed source applications applies. Natron is a dedicated compositing application that is very similar to closed source applications that do, basically, the same job, but it's not a carbon copy of them. Comp artists will be frustrated if they attempt to use Natron as a copy of other applications in the VFX field.

Natron is a fairly young application yet. While it is in use at several active production houses, it has yet to prove itself as a long-term reliable solution.