Doh! Forgot to say where to get it – prezi.com . Anyone can sign up to a free, online version, where any presentation you create is public: http://prezi.com/pricing/ .
If you can get a US .edu email address, you can sign up for an improved version (still free): http://prezi.com/prezi-for-education/ .
Are you a UK/Irish teacher? Do you use the free online version? Does it give you enough functions?