Thomas Neidhart has set up an Omnigator instance that runs on Google AppEngine, which you can visit here. He has installed the Toma implementation in the generic query plugin in this installation, which means you can try out Toma queries on it. To do so, go here, select Toma from the list of query languages, and try it out. There is a list of example Toma queries in the second drop-down list.
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.