"The Mathcounts Bible according to Mr. Diaz" is a two-page list of useful things worth memorizing, such as squares, cubes, primes, and important formulas. Start here.
"Important Math formulas" Levels 1-9, plus geometry. Each level is only 2 pages long. They start off very basic, and get more complex. These were compiled by Josh Dever. Highly recommended.
The Red Book is another source of useful formulas and information worth memorizing, this one compiled by a highly successful former Mathcounts coach who took 5 teams to Nationals. The info is good, but it's old, and the layout and typeface show it.