OptiMonk is the more common of the two in our index: it runs on 36 websites against 5 for CartStack. Counts come from real scans, not vendor claims.
CartStack is a SaaS solution that allows any company with an ecommerce site or reservation system to increase revenue through reminding/encouraging consumers to return to their abandoned cart and complete their purchase.
OptiMonk is an on-site message toolkit used to improve conversions using action-based popups ad bars.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.