ShareASale is an affiliate program founded in 2000. It focuses on small to medium-sized merchants, in contrast to Commission Junction - which focuses on larger companies. Basically for the affiliate marketer, this means that once you have set up your free ShareASale account, you have around 3500 merchants to chose to promote.
I started using ShareASale as an affiliate, by default, because one of the products I promote moved from their own affiliate program to using ShareASale's.
Totally new to ShareASale, I was rather pleasantly surprised to find that the program pays out once your commission amount to $50 and has no other fees for affiliates.