Zondle is a pretty darn impressive for online learning games.
It has tons of content in different subjects, and, if you can’t find what you need, it’s easy to just add your own. The ingenious part is that once you pick the topic you study, you have the option of studying the info in forty different games! Plus, teachers can create their own virtual classroom and track student progress. And, it’s free.
I’m adding it to three different “The Best…” lists: