Keepboard is a free open source clipboard manager that can save clipboard history (either automatically or on-demand) and provides the ability to quickly search and paste clipboard items copied in the past.
It supports text, image and file clipboard items and stores all data locally (does not require any network access nor disk access apart from its snap-confined directory). Keepboard has been tested on Linux (GNOME, KDE...), Mac OS and Windows platforms.
After it is installed and launched, Keepboard can be activated when needed by clicking on its icon in the system tray or more handily by pressing the Ctrl + ` (back tick) global hotkey (configurable in Tools -> Shortcut keys).
Note: Tray icon might not be displayed correctly on some desktop environments/themes and can be simply removed by unticking "Show icon in system tray" in the View menu.