Blabberize is great for elementary and pre-school age children and can be used for so many purposes. Designating the mouth of your speaking picture is kind of clunky, but worth playing around with. I used this site with sixth graders and their teacher for a response to literature assignment and students were asking me eagerly if we were doing blabbers the next time I came to the classroom.
The voki site can act up sometimes, but has a variety of options for customizing your avatar. It's great for all ages.
When I recently showed Toondoo to some teachers, we had trouble getting onto the site. It was a lot of fun for Solana and I, but it froze up on us several times. Some teachers commented that they don't like this site because there are inappropriate options such as bikini-clad women for some of the cartoon images.
Zimmertwins rules. The main criticism from me and teachers I've worked with is that the only choices for characters for your movie are two redheads and a cat. Nothing against red hair, we just wish students had more options to create characters that look like them. I commented to the site about this and haven't heard back.