If you put some executable text on the OS X clipboard, this can also be directly executed with the backtick operator. For example, the shell command ls -l -G -a -F $HOME produces a colorized long listing of the user's home directory. Copy that command to the OS X clipboard.
In Terminal, type pbpaste, and you will get the text of the command. If you instead type `pbpaste`, the shell will execute the command and produce the directory listing.
[robg adds: This tidbit is most useful as part of a larger script, as seen in these examples. If you're in Terminal with some executable Unix command on the clipboard, you could run that command directly by pressing Command-V and Return.]

