I got tired of tracking all my spells the old fasioned way during our Dungeons and Dragons games, so I created a spell tracker.
Available spells are placed in the appropriate spell level slots using the included tokens. Once a spell is cast, remove the appropriate spell level token and place it back in the storage area. After a rest, place the tokens back in their appropriate slot. No more note taking or (if you're old like me) writing and erasing pencil marks on your character sheet.
The lid is held in place using magnets. It uses 3mm x 1mm magnets. I paid $10 for 200 magnets on Amazon, here.
Edit: I added a base and lid that accepts a 5mm x 1mm magnet (thanks, Newfinator).
Edit 2: Blank marker added
I printed the tokens with a brim, and printed them standing up. No supports necessary.
The tokens are sized smaller than I'd have liked so that they lean away from you for easier reading when in the spell slots. If you want a more snug fit, resize the markers to 105%.