Affilo: Affiliate Marketing

Without doubt the best affiliate marketing app on the Shopify app store (and I've used them all in various capacities).

It's easy to set up and easy for your affiliates to use. That second point is often overlooked. If people can't use the software easily when they sign up, it's a barrier to them actually promoting your stuff.

If you do end up with some questions, support are great. Really quick and helpful answers.