Proofs Without Words
This is a great collection of favorite Proofs Without Words from the community at MathOverflow.net:
MathOverflow is geared toward research-level mathematics, so many of the examples deal with very advanced ideas. However, there are some lovely and accessible proofs of the sum of the first n squares, the sum of the first n cubes, the AM-GM inequality, and various Fibonacci identities.
And since it’s MathOverflow, the comments include a healthy debate about whether you can actually prove something with only a diagram!