Terra Clues is a very intriguing online geography game. It basically has a collection of scavenger hunts with clues about places the player needs to find on an online map.
A number of the “hunts” available on the regular page may be too difficult for English Language Learners, primarily because of language. However, they have recently created a separate section for teachers and students that is divided by grade level. In this section, a teacher can create his/her own “classroom” that would have “hunt’s devised by the teacher.
And, of course, students could also create their own hunts.
I’ve placed the link on my Geography page in the middle of the Sites That Cover Many Areas section.