Voki is on The Best Sites To Practice Speaking English list.
Voki has just announced soon-to-be-expanded features for the classroom, including, for a limited time, an advertising-free voki. However, it doesn’t indicate anywhere (or I just can’t see it) if there will be a cost for them. They are also developing a Voki lesson plan bank and several other teacher resources.
A Voki is a great tool for English Language Learners, and is useful — with advertising or without.