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.
Thomas Neidhart of SpaceApplications has implemented the Toma query language on top of Ontopia. Toma is a query language for Topic Maps designed by Rani Pinchuk of SpaceApplications. The implementation is currently in the sandbox part of Subversion, and not part of Ontopia proper, but the wiki page explains how to check it out and run it. Currently, Toma queries can only be run using a command-line client (or the API), but work is currently under way to make Toma available in the Omnigator query plug-in.