xclip (available as a package for Debian and Ubuntu) enables you to interact with the X clipboard directly from the command-line without having to use the mouse to cut and paste.
This is particularly useful if you're trying to get command-line output over to an e-mail or web page. Instead of scrolling around in the terminal to cut and paste with the mouse, screen by screen, you can use this:
command --arg | xclip
You can also enter the contents of a file straight into xclip:
The -o option enables you to operate it the other way around: output the contents of the clipboard straight onto the command line. So, you could, for example, copy a command line from a web page, then use
xclip -o /path/to/file
Use the -selection switch to use the buffer-cut or one of the other selection options, rather than the clipboard default. You can also hook it up to an X display other than the default one (e.g., if you're logged on as a different user on :!) with
xclip -d localhost:1