Stig Lau has set up an example Ontopia application (source) running on Google AppEngine. This means that Ontopia web applications can run on Google AppEngine, which again means that it’s possible to get free hosting (up to a certain limit) for Ontopia-based web applications.
Note that there are some limitations. Firstly, this does not apply to the 5.0.0 release; Google AppEngine only works as of revision 421. Secondly, it only works with the in-memory backend for now. We are considering to try running with a pure-Java database to see if that is possible, in which case this could become a very interesting option indeed.