While I was preparing for Network+ I bought a 2x4 Whiteboard on a whim and I think it might be one of the best decisions I have ever made. I am able to draw diagrams, write out terms I am studying, and let concepts just sit on the board for a couple days to let them sink in. I am lucky enough to work from home so my study space and my working space are the same. Another tool I use is Anki, it is the best flash card program ever, I even purchased the $20 iOS app (mainly to help support Anki). Anki gives you options to see flash cards again in 4 days, 1 day, or 10 minutes to help get the term in your brain. The more time and effort you put it the more you get out.
https://ankiweb.net/
What are some invaluable tools that you have found extremely helpful in learning new concepts? And I am not just talking about tools to pass Certs but tools that help you learn new difficult concepts.