Open Source 2D Animation Tool - Synfig
I came accross this open source 2D annimation tool called Synfig. It's still in beta phase, but it looks to be pretty interesting. One of the things it's supposed to be really good at is tweening. It's intended more for Film and TV type annimations, rather than typical net stuff. I'm not sure if it's going to be as good as After Effects or Flash for most things, but it could prove useful for something!