Also, http://www.minuscule.tv/ Minuscule 5-minute series of bugs comes up as a very good example of 3d animation on 2d background (plus one fancy homepage).
I think http://www.lotte-film.com/ Lotte leiutajatekülast/Lotte from Gadgetville has already been mentioned. There is an English and German versions of the page too, but they are more like flyers, so I gave Estonian one. I would still note that this movie is only one of the great works by the http://www.joonisfilm.ee. They are number one animators, when making movies with details that make you watch the movie again and again.
http://www.youtube.com/watch?v=NzxSa4ZULBwLes Chasseurs De Dragons/ Dragon Hunters is also something to see. The link will lead to youtube, where there is only a song up (anyone, who knows a better link, PLEASE let me know.
Didn't manage to go through all of the topics, so I apologise if I reposted any.
Have fun 