I am looking to implement a "refer a friend" program with campaigns such as "Give $20, Get $20". What software platform do you use or recommend?
Some of those built their own in-house. But some platforms that many companies use are: Rewardstream, Referral Saasquatch, Referral Candy, and Amplifinity.
When we did this for our own company we implemented in house, but we were at a high level of execution and resources (<100 employees).
If you are small I'd recommend http://www.referralcandy.com/. It's a very simple system that you can implement and the payment tiers let you get it fairly "invisible" if you so desire.