I used Experiture for just over a year and they were so-so, not slick and sexy like some other UIs. They're also not as cheap as some of the big guys like Constant Contact or SendGrid. Deliverability became an issue for us so we had to find a new service.
Best Referral Programs - A curated directory of startup referral programs
eTrigue - Real-time Sales Acceleration & Marketing Automation Solution
Viral Loops - Viral Loops is a viral and referral marketing platform to launch ranking competitions, sweepstakes, pre-launch and referral programs.
Pardot - Salesforce B2B marketing automation
GrowSurf - Referral software for B2C, FinTech, and SaaS companies.
Campaign Monitor - Email marketing software built for designers and their clients to run successful email campaigns.