This virtual manipulative allows you to display, rotate, and resize Platonic solids. It also allows you to select vertices, edges, and faces, and observe that the number of vertices minus the number of edges plus the number of faces is always equal to 2 (Euler's formula).
http://nlvm.usu.edu/en/nav/frames_asid_128_g_1_t_3.html?open=instruction...

Attribute blocks can be used to help students understand colour, shape and size. This is a lovely Java implementation of this manipulative.
http://nlvm.usu.edu/en/nav/frames_asid_270_g_1_t_3.html?open=instruction...

Algebra tiles are used to help students understand expanding and factoring quadratic epressions. This is a nice Jave implementation of this manipulative.
http://nlvm.usu.edu/en/nav/frames_asid_189_g_1_t_2.html?open=activities&...

This J virtual manipulative uses plus-minus chips to demonstrate adding positive and negative values.
http://nlvm.usu.edu/en/nav/frames_asid_161_g_1_t_1.html?open=instruction...

The goal of this Java puzzle is to put three numbers inside of each circle so that they add up to 99.
http://nlvm.usu.edu/en/nav/frames_asid_269_g_1_t_1.html?open=instruction...

This virtual manipulative allows the user to visualize the product of two integers.
http://nlvm.usu.edu/en/nav/frames_asid_322_g_1_t_1.html?open=instruction...

This virtual manipulative allows the user to visualize the product of two factors in three different ways: click on any of the options below the array to choose Grouping, Common, or Lattice.
http://nlvm.usu.edu/en/nav/frames_asid_192_g_1_t_1.html?open=instruction...

A nice Java implementation of the classic Mastermind game.
http://nlvm.usu.edu/en/nav/frames_asid_288_g_1_t_1.html?open=instruction...

A visual way to teach percent operations is the 'TopHat' - which shows the relationship between the Part, the Percent and the Whole. This Java applet uses proportions to set up and solve percent problems.
http://nlvm.usu.edu/en/nav/frames_asid_160_g_1_t_1.html?open=activities&...

This virtual manipulative can be used to visualize a sequence of operations on a number line and to create number sentences representing the operations.
http://nlvm.usu.edu/en/nav/frames_asid_107_g_1_t_1.html?open=instruction...