RetroTool is a simple and flexible tool for online collaboration, especially retrospectives! You create a retrospective, add columns you need (you can also use their templates) and share the link with your colleagues, so they can join. Afterwards each of you create their own cards in Private Sections (so you avoid biases) and when everyone is ready you publish them by either dragging your cards and dropping in a column or using the "Publish all" button in the top left corner of each Private Section.