Which affiliate program should I choose?

I have a website in development at this time. I will be looking for affiliates to assist in the sales of the service that the website offers. I want to find an affiliate program which only pays the affiliate after an actual sale is generated.

With this in mind, what are the top few programs that I should be looking at?

I remember seeing one which was well respected, but I can't seem to remember the name of it for the life of me.

Thanks guys