The simplest way is:
cat > new_file_name
Then paste in your text.
Press control-D on a blank line to finish.
If you want to edit anything on a previous line, though, you're out of luck. You'll have to use a real editor for that.
This is the first unix system I've been on that doesn't have the standard 'head' command to view the top of a file. Instead, the head command on OS X does an HTTP head request. Is there some other head-like binary, or do I need to roll my own?