Sailthru (https://www.sailthru.com/ https://www.sailthru.com/) is a good one. API is easy to use and they offer a level of analytics that is far more advanced

Sailthru (https://www.sailthru.com/ https://www.sailthru.com/) is a good one. API is easy to use and they offer a level of analytics that is far more advanced than what most other companies can offer. Disclaimer: I am not a Sailthru employee but I have done a number of integrations for clients of theirs.