Depends if you're looking for anything specific..One of the better affiliate sites that I personally get paid from on time every time is
CPA Prosperity which handles a bunch of affiliates..They have everything from free offers like free xbox's and gift cards to free dating signups, to paid offers for things like credit reports and work from home. Payouts per lead range from like $1 on a free download offer up to like $36 on a credit card offer..
It all depends what kind of affiliate you need since mainstream is so big..If you could be a little more specific I may have some other trusted suggestions, but these guys are great because they approve quickly, pay on time, and offer a variety of programs from one spot.