Free, Open source, and just enough node-based compositing software in the open-source eco-system, Natron, is no cut-down version of a full-fledged compositor. It is a full-blown compositor, following in the footsteps of industry standard, commercial composting applications. It's got enough nodes to get you through your daily CG as well as film compositing needs. Also, it uses shortcuts identical to Nuke. The minimal learning curve for users already familiar with Nuke.

It is free software with big opensource OpenFx effects. The UI similar to Nuke. Users engage in open discussion about new tools with developers. An additional advantage of Natron is the availability of a version for 64-bit systems, as well as portable assemblies that do not require installation and can work from any carrier! It can render several graphs at the same time, thereby speeding up the delivery of the final result, as well as use a distributed render using computers on the local network.

I like the user interface's similarity with Nuke. Makes using it non confusing as only a very few number of tools differ in name. Everything else is pretty much the same.