Ktoon seems to be mainly for Linux. I went to their website which seems to be mostly in Spanish you can click the English tab, however it seems several languages were mixed together, which made reading the website hard. I went to the user video example gallery and there was 1 video posted. Now if the software was easy to use, you would think there would be a large number of videos from users, since after all it is free. I also searched youtube for examples and I did find a few, mostly stick men type animations. I did not find very many tutorials either, so I will pass on this one.
Synfig looks promising, however doing research I notice that it does not handle sound. Now for music and such that should not be a problem, however for lip-sync I would think it would raise a lot of problems. How can you animate the lip-sync without being able to work with it in the animation program? Synfig had much more examples, both on youtube and on the main website.
Below is a Synfig eye animation, I think it looks very good. However, not having audio in the program makes me Leary.
Pencil seems to have a pretty good following. However, pencil is a frame by frame animation software. I was hoping to use the power of the computer to do the in between frames. So I will keep looking.