Here are the most useful resources I've found and used in my years teaching middle school math. I haven't taught full time math in a few years so maybe some are outdated, but hopefully there are some that are still useful.
General math resources/Tools
Desmos Calculators and Tools - Best online calculators. I was pretty excited when our state test switched to using their tools. Small thing, but it does make a difference to have students familiar with the tools.
LabCamera - A great tool for using the camera to graph movement or take time lapse videos. Very science heavy, but graphs and data included.
Logic Puzzles - There's a collection of websites that have around 35 different logic puzzles, but each puzzle has it's own URL. Just scroll down to see the other options. I love this site because there's a "mass print" option on the left side to print full pages of them. I've used Shikaku and Nonograms a lot in my classes. Shikaku is great practice for breaking numbers down into their factors.
PhET Math Simulations - I use PhET simulations in science a lot, but they have a whole section of math simulations. Helpful when trying to visualize math concepts or working online/remotely with manipulatives.
CK12 - Free curriculum to use all or part of. Lots of options!
Banzai! - Free financial math curriculum at multiple levels. You can download PDFs or get books.
Mammoth Math - Not free, but the least expensive 1st-7th grade math curriculum you'll ever find. PDF downloads you can save and print whenever you need. I would use this with my students needing extra practice. PDFs are organized by grade and topic making it easy to find what you're looking for.