Does anyone know of a robust compositing application which is open source more or less like Blender? Would be grateful for any info.
No open source compositor as good as Blender exists.
Those that do are…
- Jahshaka/Cinefx: Dead.
- Ramen: Slower than a pig with no legs.
- Synapse: See above.
Try Blenderguru’s tutorials. It uses Blender’s internal compositor (which is more or less like Blender), and you’ll learn to do amazing things. 
For compositing, Blender is pretty good…