what are the best top affiliate networks or sites to make money from?

i want to know whats the best affiliate program there is to promote for….you know like sign up with them and promote there site and they give you a unique link so you can post it on your website and they will pay you….you know like your spreading the word for them and getting a small compensation……kind of like a refer a friend program……….any FREE to sign up legit ones out there and any worth it?