While not technically a web tool, this free download allows students to create, alter, and run engineering simulations. Students can change gravity or add gears, planes, ropes, and wheels to see how they will interact--they can even add drag and friction. This is a very fun tool all around, especially when thinking about simple machines. It would fit well with adventure or chain reaction novels.

This simple block coding program uses code to make 3-D objects. With standards-aligned lessons, this is an amazing resource for teaching math concepts. The lessons are included in the premium membership, but the tool is free. All files also export as an SVG, so printing them out on the 3-D printer is a breeze.

The CoBuild website and Facebook group offer a wealth of online maker activities. Even better, they include concepts from luminaries in the making world such as Adam Maltese, Erin Riley, and Joel Bruns, and compile work from the Amazeum, the Tech Collective, and other sources.

This resource is low on technology but high on engagement and reliability. All the projects can be made with low-cost or recycled materials. The bagpipe is particularly fun, as is the unique take on the spaghetti/gumball project that involves balance. Check out the project library for STEM-rich ideas.

The well-organized site from San Francisco's Exploratorium investigates everything from skateboard engineering to cookie subduction (i.e., using an Oreo cookie to demonstrate what happens when continental and oceanic plates shift). It is heavily science-focused and follows the scientific method closely. 

This archive of experiments is a rabbit hole of all things awesome. Curated into collections, the experiments are easy to use and share, require little to no equipment, and are deeply entertaining with a firm curricular foundation. 

Microsoft's MakeCode program includes both physical computing with the Micro Bit and virtual coding and app development. Easy to use with limitless possibilities, it's a great block coding interface. It even has Minecraft coding!

Maker Ed has compiled an archive of activities with a robust pedagogical and curricular foundation. It is easy to search and use but dense with academic language.

The maker challenge archive from Science Friday is extremely robust. Everything from "The Many Types of Mucus"  to "Fossilize Me" card game is available here. Key vocabulary words are bolded, and the clear instructions are written with students in mind.

No list would be complete without this perennial favorite. Now in partnership with the Harvard Graduate School of Education, the site includes an archive of activities for Scratch that has hundreds of math and visual arts activities, and examples and stories to help build classroom community and support curricular content.