xcutsel
A small X program with two buttons that shuttle text between the PRIMARY selection and the old cut buffer, for programs that only understood one of them.
X ran two generations of clipboard side by side. The older cut buffers, eight properties on the root window that programs simply wrote text into, were a holdover from X10, while newer programs used ICCCM selections. A program of one kind couldn't paste what had been copied in the other. xcutsel bridged the gap with two buttons: one copies the contents of PRIMARY into cut buffer 0, the other takes the cut buffer's text and makes xcutsel itself the owner of PRIMARY. -selection picks a different selection.
It shipped with the X distribution alongside xclipboard and stands as a monument to a transition that was never quite finished. It is still around, and anyone who digs up a sufficiently old program will still find it useful.