Moosend is very popular. They advertise themselves as an all-in-one marketing platform.
In addition, they have email automation tools and drag & drop landing page builder.
Pricing starts at $10/mo. They also have a free tier allowing for up to 1,000 email subscribers.
The commission you can earn from each referral can be pretty high thanks to that it is recurring for as long as he or she remains a paying customer.