The free site has word lists that have supposedly been designed for grades two-to-twelve. It has audio and image support for the text — in fact, it would be pretty difficult to come-up with a “definition” design that’s more accessible to English Language Learners.
Unfortunately, though, the word lists themselves do not appear thematically designed. In fact, for the ones I looked-at (and I only spent a little time at the site) I couldn’t discern what logic they used to devise the lists (obviously, they must have used some kind of system, but it certainly wasn’t designed for English Language Learners). The associated games didn’t appear particularly impressive, either.
That being said, it’s still definitely worth a visit. The word definitions, as I said, are extremely accessible. I’ve placed the link on my website under Vocabulary.