1. Diamond: A crystalline form of carbon with a very rigid, three-dimensional structure. It's incredibly hard, transparent, and has a high refractive index, making it valuable as a gemstone and industrial tool.

2. Graphite: Another crystalline form of carbon but with a layered structure. The layers are held together by weak forces, allowing them to slide past each other, giving graphite its lubricating properties. It's also used in pencils and as an electrode material.

3. Fullerene: A class of carbon molecules that form closed, cage-like structures. The most famous fullerene is buckminsterfullerene (C60), shaped like a soccer ball. They have unique properties and potential applications in nanotechnology and materials science.