Interactive
National Council of Teachers of Mathematics

Nctm: Illuminations: Cube Nets

For Students 4th - 6th
Interactive site where students find which nets are able to form a cube.
Activity
University of Cambridge

University of Cambridge: Nrich: Instant Insanity

For Students 3rd - 8th Standards
Excellent interactive challenge working with cube nets to stack them in a specific order. Manipulate the cubes online to solve the problem.
Interactive
Other

Calculator.net: Surface Area Calculator

For Students 6th - 8th Standards
Use these calculator tools to find the surface area, in your choice of units, of a sphere, cone, cube, cylinder, rectangular prism, capsule, ball cap, conical frustum, ellipsoid, or square pyramid. Includes a reference section listing...
Activity
University of Cambridge

University of Cambridge: Nrich: Making Boxes

For Students 3rd - 8th Standards
Have fun and learn by making a box and then find out its maximum volume as well as other problem-solving activities, including making a table of your results and drawing a graph.
Activity
National Council of Teachers of Mathematics

The Math Forum: Polyhedra Nets Paper

For Students 9th - 10th Standards
Contains six examples of polyhedra nets (cube, tetrahedron, octahedron, dodecahedron, icosahedron, cubosctahedron).