Awesome

clipnotify is a simple program that, using the XFIXES extension to X11, waits until a new selection is available and then exits.

It was primarily designed for clipmenu, to avoid polling for new selections.

Here's how it's intended to be used:

while read; do
    [an event happened, do something with the selection]
done < <(clipnotify -l)

Or:

while clipnotify; do
    [an event happened, do something with the selection]
done

clipnotify doesn't try to print anything about the contents of the selection, it just exits when it changes. This is intentional -- X11's selection API is verging on the insane, and there are plenty of others who have already lost their sanity to bring us xclip/xsel/etc. Use one of those tools to complement clipnotify.

You can choose a particular selection with -s, and loop instead of exiting with -l. See clipmenu -h for more information.