I would say website.ws is the best, it has a referral system really nice, for every person you referr, you get 1 dll, but not only do you get 1 dll for the ppl that you refer, but if the ppl you refered refered someone, it will also count as your refer, and so on till 5 levels, so for example:
you refer 5 ppl, and each refers..3 ppl, that would be 20 dlls :O
check it out, here is the link with a nice video:

freedom.ws/cananito