Natron
I remember when I first started studying computer graphics back in 2009, I was introduced by someone to Blender. I remember searching a few months later for an equivalent (dedicated) open-source compositing package, to no avail.
For years I resigned myself to such a thing simply not existing. Then Blackmagic bought Fusion!
Now I know it's a far cry from open source, but it's both $0 and awesome. However I stumbled upon this program today that's an open source, node based compositor.
I'm not sure how long it's been around for, but it's really great to see something like this gaining traction, so I thought I'd put it up here and help them gain some free publicity :-)