For key-frame based animation, we got Synfig which is cool, but do you know any good software for “classic” (frame by frame) animation? Sure, we can go and use GIMP + layers + layer transparency (for simulating “onion skin”), but it is very irritating and gives classical animator another burden (hand tweening is already hard enough).
Preferably applications that are free (not necessarily open source though).
//edit: And if anyone wonders: Yes, hand tweening is easier for me than using synfig and have some successes with classic animation (though it is either too brutal or NSFW in general too show it off on site like this where kids my watch it).