It's time to announce testing for a further two Glk subengines. The first is the Glulx interpreter, for which there are many free games available at the IF-Archive.
The second is the Comprehend engine games developed by Polarware. Here's where things get a bit more complicated. At the moment, only the first three Comprehend games are ready for testing - Transylvania, Crimson Crown, and OO-Topos. These three are also freely available at the IF-Archive. There is also, however, a re-release of Transylvania also available for free from the Graphics Magician website which isn't yet supported.
As for the other Comprehend games, Talisman will likely be supported in the future, but it requires significant further work. The other remaining games like Transylvania III use a completely different engine, so will remain out of scope for the foreseeable future.