A TI-Nspire activity for investigating opposite angles and angles formed with two lines are cut by a transversal.
http://www.tigeometry.com/activities/cabrijr/239/

Students use a TI-Nspire calculator to investigate coordinates of points and the gradients (slope) and equations of linear functions.
http://www.tialgebra.com/activities/algebra_i/335/

There are some mighty fine tutorials on the web to help teachers and students learn more about this amazing technology.

Getting Started with TI-Nspire
A T-cubed on-line course on the TI-Nspire.
http://education.ti.com/educationportal/sites/US/sectionHome/pd_onlineti...

Atomic Learning
Atomic Learning is an award-winning provider of web-based software training and tutorials.
http://movies.atomiclearning.com/k12/ti_nspire/

A snazzy virtual geoboard. You can teach just about any topic with a geoboard. If you don't have a class set of the real thing, get one!
http://nlvm.usu.edu/en/nav/frames_asid_125_g_1_t_4.html?open=activities&...

With this virtual manipulative you can create objects with pattern block pieces and explore Translations ("Slides") as transformations of the plane.
http://nlvm.usu.edu/en/nav/frames_asid_167_g_1_t_3.html?open=activities&...

With this virtual manipulative you can create objects with pattern block pieces and explore Rotations ("Turns") as transformations of the plane.
http://nlvm.usu.edu/en/nav/frames_asid_207_g_1_t_3.html?open=activities&...

With this virtual manipulative you can create objects with pattern block pieces and explore Reflections as transformations of the plane.
http://nlvm.usu.edu/en/nav/frames_asid_206_g_1_t_3.html?open=activities&...

Choose some shapes and try to tesselate the plane with them.
http://nlvm.usu.edu/en/nav/frames_asid_163_g_1_t_3.html?open=activities&...

Tangrams are a versatile manipulative. They can be used to build understanding of geometry, fractions, area and other topics. This is a Java implementation of tangrams.
http://nlvm.usu.edu/en/nav/frames_asid_268_g_1_t_3.html?open=activities&...

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...