CopyQ is clipboard manager – a desktop application which stores content of the system clipboard whenever it changes and allows to search the history and copy it back to the system clipboard or paste it directly to other applications.
Diodon is a simple Gnome clipboard manager. It provides app indicator support. And strives to be tightly integrated and easy to use. It still comes with a lot of customization and security options.
Parcellite is a X11/Gtk+ clipboard manager. It's lightweight and with a small memory footprint, by providing mostly basic functionality. Parcellite keeps a history of X clipboard contents, provides some view options, retains saved text even after application closure, provides a systray icon and global hotkeys for item access, but also allows to define custom commands using clipboard contents.