xclipboard allows you to save, load and alter multiple 'CLIPBOARD' selections under the X Window System, as per I39L. It is one of the core programs included in the X11R6 distribution from X.org, along with xload, xterm, xman and others. The source for xclipboard is written in C, Xt and Athena; setting appropriate resources in .Xresources or xrdb lets you customize it. I should note that xclipboard, like all Athena programs, is absurdly ugly in comparison with programs written in GTK+ or QT, unless you're using Xaw3d or some such.
Anytime a selection is posted to CLIPBOARD, xclipboard grabs it and puts it on top of it's stack. It keeps all grabbed selections until it is closed or they are manually deleted. You can save the selection to a file, cycle between all existing selections, or load a selection from a file for export into another program using the obvious buttons at the top. You can alter the current selection by changing it in the textbox. xclipboard is simple, visually compact, and generally useful for those not using a heavily integrated environment.
For me, xclipboard is handy because I don't use a popular desktop, preferring evilwm and the command line. I recently used it to copy a length of source code from Mozilla Firefox into a plaintext file. Before, I had been using gvim (Not being a believer) to do so; xclipboard has made that part of my life a little easier.
Like all Xt programs, xclipboard accepts the "-geometry WxH+X+y" options, and many others.