My students, for the most part, really liked the Scatter game on Quizlet --- I could walk around and see how students were doing and announce the fastest time within a section and among all my sections. If you put enough options in the Quizlet set, students won't necessarily see the same questions every time, so they really need to pay attention. The only negative I saw was that some students would just tap on squares as fast as possible to get right answers by accident. (If you want, you can see my Quizlet sets at https://quizlet.com/drmarkschlatter.)
It turns out Desmos has a relatively new feature called card sort that carries out some of the same functionality. You don't need to have one to one matching, so I tested the feature by creating an activity that assesses whether students can classify a number as an integer or a rational number. Here's the screenshot:
Unlike the iPad version of Quizlet, you drag the boxes together to form groups. You can specify an answer key and see which card were mis-sorted most often (that's a really nice feature!). I'd rather do this on a Venn diagram somehow, but this is a very good second choice. (You can find my activity here.)
I think I could use Desmos card sort for most of the material in 6th grade that Quizlet works for and (as shown above) maybe a bit more. A really good use would be to create cards that show the different ways ratios can be expressed (e.g., rate table, equation, graph, and words.) I'll have to work on that!