Dropbox

Dropbox's defining move was making the cloud invisible: a folder in your file system with a green check that meant 'safe.' That status badge taught a generation what sync feedback should feel like. The desktop client still leans on it, surfacing per-file progress and selective-sync toggles where they matter. Over time the web app accreted previews, comments, and dashboard cruft that fight the original simplicity, and the menu-bar app now nags more than it informs. But the core interaction holds: drag a file in, watch it propagate, trust the checkmark. Version history and file recovery are excellent and underplayed, tucked behind right-click. When it stays out of the way, it's the cleanest sync mental model in the category.