I have quite a few sites on my The Best Sites That Students Can Use Independently And Let Teachers Check On Progress list.
I’m adding two “sister sites” to it — Qwizzy’s World (for K-7) and Cram Stoppers (for 8-12). Both only provide quizzes, so they are much more limited than most of the other sites on that list. However, unlike many of the other sites I have posted about, at least the Cram Stoppers might be usable for mainstream higher grades.
Because of their limitations, however, I wouldn’t use it during the school year. I’d just include it as an option for students to use during vacations to limit the summer slide.