Visitorboost.com is one of the first affiliate sites I joined, and I'm still a member today. They have a very simple interface, packages starting at less than $2.00, and a very quick turnaround time for approving sites. I'd recommend this site whether you're just starting out or are a veteran.
A second site that is closely linked to visitorboost is shareasale.com, which in my opinion rivals some of the giant publisher/affiliate networks out there (adzoogle,commisionjunction, etc) in terms of polish and the amount of resources available for both affiliates and publishers. Like I said at the start of the post, these two sites have helped me immensely, and I give them a solid 8.5/10.