Many choose ClickBank for low ticket digital products simply due to it's simplicity ... though they are neither cheap nor perfect. Same with PayDotCom.
Paypal as a processor is fine for single tier promotions that are free from any red flags that might trigger a freeze ... once again, not cheap nor perfect ... but so easy.
1SC is obviously a choice, though their autoresponder and affiliate modules leave much to be desired ... GoldBar One is a worthy alternative.
Frank Kern, Yanik Silver, Jeff Walker, Mark Joyner and others of repute count on Paul Galloway and his Synergyx script ... sometimes in conjunction with 1SC and AWeber.
I've always liked Ultimate Affiliate ... though it has not been updated in years. Jeremy and Simon of Marketing Main Event have a modified version of it I like.