Online Geometry, See also:

Ceva's Theorem. Concurrency. Interactive proof with animation.

Puzzle of the Ceva's Theorem: 48 classic piece.

Pascal's Mystic Hexagram Theorem Proof

Blanchet Theorem

Nagel Point Theorem. Proof.
 

Nagel Point Theorem - Flowchart Proof .
 

Gergonne Point Theorem. Concurrency. Interactive proof with animation.

Key concept: Ceva's Theorem.
 

Triangle with the bisectors of the exterior angles. Collinearity.Key concept: Menelaus Theorem.
 

Monge & d'Alembert Three Circles Theorem I with Dynamic Geometry

Monge & d'Alembert Three Circles Theorem II with Dynamic Geometry

Newton's Theorem: Newton's Line. Circumscribed quadrilateral, midpoints of diagonals, center of the circle inscribed.

Puzzle of the Newton's Theorem: 50 pieces of circles.

Pentagons & Pentagrams. I found, exploring with dynamic geometry software, new facts about pentagons and pentagrams. Key concept: Menelaus Theorem.

See also: Pentagons & Pentagrams - Puzzle: 96 triangles.

Van Aubel's Theorem II: Triangle and Cevians.
 

Eyeball Theorem: Animated Angle to Geometry Study.

Geometry Problem 659.
Parallelogram, Parallel Lines, Collinear Points.