Euclid's Orchard serves to provide high quality math handouts. 

One Page Summaries

A short compilation of topics in contest mathematics to study.

Written by Dylan Yu and Amol Rama.

Recursion in the AMC and AIME

A nice introductory handout to the idea of forming and solving recursions.

Written by Jeffrey Chen and Dylan Yu, edited by Peter Pu. 

Modular Arithmetic in the AMC and AIME

Goes over most concepts in modular arithmetic, although some overlapping topics with other parts of number theory may have been left out.

Written by Dylan Yu. 

Trigonometry in the AIME and USA(J)MO

A complete guide on how to use trigonometry on the AIME and USA(J)MO. Includes (almost) every trigonometry problem on the AIME, with worked out problems as well as more than one hundred hints to selected problems. There quite a few olympiad problems sprinkled in, too.

Written by Dylan Yu and Amol Rama.

Polynomials in the AIME

A complete guide on how to use polynomials on the AIME. Includes (almost) every polynomial problem on the AIME and also problems from other sources (such as RMO and HMMT).

Written by Dylan Yu and Amol Rama.

Proofs in Pre-USA(J)MO

Introduces proofs very nicely.

Written by Jai Sharma.