Well known math competitions:
Mathcounts

AMC 8 – January

AMC 10/12/AIME/USAMO – Nov/Feb/March

ARML – Early June

USAMTS Year Round

Less well known contests that I like:

OMMC – Online Monmouth Math Competition – May to June

Purple Comet-Team competition of about 4 -6 students per team. – April

Good past competitions:

ITest

Mandelbrot Competition

Other Contests:
Mu Alpha Theta

Always compete in your local math contests!