RoundCube is the more common of the two in our index: it runs on 28 websites against 5 for Zadarma. Counts come from real scans of each site's home page, not vendor claims, and cover the sites in TechLookup's index rather than the whole web.
RoundCube is free and open-source web-based IMAP email client.
Zadarma is a VoIP service that provides cloud-based voice communications for businesses, supporting internet calling, virtual phone numbers, and call management features.
Within the webmail sites we have indexed, RoundCube holds 0.0% and Zadarma 0.0%.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.