2D ANIMATION program

There are plenty of programs like Gimp and Inkscape that can be used for single images, but is there any program similar to anim8or that will allow me to choose a beginning frame and an end frame and have the program fill in the middle? Does Gimp already have this function and I just can't find it? feral made something like what I want but I don't think he ever finished it. Do you guys just make sprites one at a time?