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.
http://natron.fr/
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 :-)