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Maybe it's worth mentioning that Clipboard is just using xclip under the hood (on unix-ish systems). In other words, xclip would need to be installed for it to work... — which unfortunately often isn't the case, as xclip doesn't belong to the base tools that come with X11.
In some cases, you might find xsel installed instead, which is another similar utility.
BTW, seems like a good occasion to advertise a little trick: you may use xclip to conveniently copy files between machines when you already have a working X connection (e.g. ssh login with X forwarding enabled), and other means like scp or s/ftp would require you to re-authenticate.
For example, when I log into client sites, I often have to go through a somewhat cumbersome procedure, requiring me to enter various passphrases, SecureID tokens and whatnot, until I finally have my SSH/X connection... (yes I know there's ssh-agent, but it doesn't always help). In these cases, xclip comes to rescue. I.e. with the following two aliases in my ~/.bashrc (though it of course also works without aliases):
I can just type
on the local machine. And then, on the machine I'm logged in remotely:
to have the tarball transferred without needing to retype my credentials every time.
(But mind you, however neat this seems, it isn't the right tool for copying Gigabyte-sized volumes... :)