Optimizely is a lower cost entry tool for anyone looking to do website testing. While their pricing structure has changed over the years it still is a pretty cost effective solution. It is easy to get tests up and running within 5 minutes once you have code placed on your site.
I love the drag and drop features of Optimizely. It makes it very easy to make simple changes to do landing page testing for my paid search campaigns. It also frees up dev time for our web team. Also, The findings from Optimizely tests are so valuable for an end customer experience.
PartnerStack - GrowSumo helps growing companies increase sales, signups, and leads through partnerships Whether you're starting fresh, migrating a partner program, or ready for hyper-growth - we're ready, are you?
VWO - Use VWO to tweak, optimize & personalize your website with minimal IT help.
Invite Referrals - Invite Referrals is the simplest tool to design and launch customer referral campaigns.
Google Analytics - Improve your website to increase conversions, improve the user experience, and make more money using Google Analytics. Measure, understand and quantify engagement on your site with customized and in-depth reports.
Impact - Impact Partnership Cloud is an affiliate marketing tool that helps businesses to manage the entire lifecycle of partnerships from recruitment to incentivization.
AB Tasty - AB Tasty is an all-inclusive platform for conversion rate optimization, personalization, customer activation, and testing.