Of the layering apps I review, Hypothes.is seems to get the most online attention. Here is another recent post offering background on the product, providing a basic tutorial, and supplying some suggestions for classroom use. I don’t necessarily consider Hypothes.is the most complete service, but it is free and seems to be the most common from the layering category of services used in higher education.
My list and description of other services within this category can be found at Layering for Learning.