Clipman-plugin
sudo apt-get install xfce4-clipman-plugin
No config function. No hotkey. Very basic function. But it support 1-step-paste.
Parcellite (1.0.2rc5)
Installation with apt-get is different with that with Software Manager. That from apt-get seeams working strange, so not use it. Use Software Manager to install it. A GUI and you can config its hotkey. But its a 2-step-paste app (you have to select a item using hotkey, then focus target text editor, then ctrl-v to paste). Parcellite can enable "copy on selection" by checking "Use Primary" and "Synchronize clipboards" in [Preferences -> Behavior -> Clipboards].
But in GUI this feature always causes trouble. For example, we want browse website yahoo, so we first copy text "www.yahoo.com", then switch to browser, select all text "www.baidu.com" in address bar, then overwrite them with shortcut Ctrl-v. With "copy on selection" enabled, when you select "www.baidu.com", it's automatically copy to clipboard. So when you press Ctrl-v, you get "www.baidu.com" instead of "www.yahoo.com".
In console environment "copy on selection" is convenient, but with tmux, it's unnecessary any more.
Glipper (2.4)
Install with Software Manager. Behave like Parcellite, you can configure hotkey, but 2-step-paste.