Added CopyQ in "alternatives" section, thanks!
https://github.com/naotaka/ClipMenu
It's a bit of a Swiss Army knife for the clipboard, and among other actions, allows for user-created Javascript to act upon clipboard contents. I don't know if the scripts can be called automatically, however.
http://www.clipmenu.com/static/images/screenshot/en/pref-act...
FWIW, the documentation says it's for OS X 10.5-10.6, but I have it running on 10.9 without issue.