The best alternatives to Marker are Trello, Codesign.io, and Marker.io. If these 3 options don't work for you, we've listed a few more alternatives below.
Trello is a web-based Kanban-style list-making application. Originally made by Fog Creek Software in 2011, it was spun out to form the basis of a separate company in 2014 and later sold to Atlassian in January 2017.