Boundless is another example of high quality Open Educational Resources (OER), like some others that I’ve written about here. Like many of the others, too, it is growing in the number of subjects it offers, and that list is already substantial. Boundless textbooks are totally free, are available across several platforms, and have a very nice built in reader that allows you to take notes, save your progress, highlight important text, and list questions that may come up as you read.
Project Based Learning: If you have students venturing out for some self-directed learning, but less focused on a traditional driving question (like a good project is), then this is a high quality resource. Certainly online learning takes a lot of discipline and you have to work out with the student how to define the scope and plan of such a project, but with that in mind, this would be a great place to look for resources.