Here is a tool to partly replace the video commenting function in the soon defunct Vialogues:

Here is the full article by Taibbi. I hope it is open for all.

I also used this to test Screencast-0-matics new tool ScreenPal. You can comment on anything although it is not as fine-grained as Vialogues. In other words you cannot hightlight a particular timestamp to comment upon.