Q:

A command that gives username@hostname:pwd

I know that pwd gives the current working directory, hostname gives the current host and whoami gives the current user. Is there a single unix command that will give me the output of
whoami@hostname:pwd

so that I can quickly paste the output into an scp command?

A:

Not a single command as far as I know, but this does what you need:
echo "$(whoami)@$(hostname):$PWD"

You could make that into an alias by adding this line to your shell's rc file (~/.bashrc, or ~/.zshrc or whatever you use):
alias foo='echo "$(whoami)@$(hostname):$PWD"'

