I've come across these two separate open source programs that deserve some attention. First there is Natron, a nodal film compositing software that closely mirrors the Foundry's Nuke. It has access to many OpenFX plugins and exists to give students a way to learn the standard compositing workflow.
The other contender is a painting program, Krita, and I'm quite enthused to give it a go and see what it offers, it has a different take in that it is not a Photoshop clone, it's something else.
Of course, as is often the case, these being free, they'll get downloaded, perhaps installed, but will they get used?