Flexible online textbook-- lots of resources!
I use this website as a resource when I'm searching for background information on a particular topic. I would say that 80% of the things I've accessed have been of very high quality. Sometimes there are videos on there that an average YouTube.com search would not find. All of the videos have been vetted by other users, but that doesn't necessarily mean that they're great.
How I Use It
I use it as a way to build my own understanding, fill in gaps in background knowledge, etc. I access it when I want to review a particular topic. I also recommend certain pages of it to my students if they've been absent and missed a particularly complex concept or when their quizzes show that they need to brush up. I recommend it as a resource or a place to begin online individual research.