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.