I just find out there is a shortcut key for display the top 5 entries of system clipboard, it's Ctrl-; .
You can use push Ctrl-; in gedit, terminal or even in web browser, to get the top 5 entries of you clipboard content and press num 1 to 5 to paste the content. It's terrific.
I just want to know which application provides this function and the setting place.
B.T.W., I don't find the setting in System Setting -> Keyboard -> Shortcuts