As a developer, every time I find a problem I first seek to solve it myself than using other tools. I'm stupid. I spend some hours fighting with a problem and at some point I realize about the real difficulty but still I'm hard-headed and don't want to pay for a service. Thankfully, I was too lazy to build my own referral system and stumbled upon Tuemilio. I've already used it in two sideprojects and it's helped me discard and validate both ideas. I really recommend to set it up early on! Also, it's a bargain!