rsync

As part of transitioning from using an old desktop to a new desktop (while both are still active), I found the 'rsync' command to be very useful.  Here is the options I ended up using to accomplish my goal of copying over (old) files from the old machine to the new without wiping out any new files on the new machine:

rsync -avzuhP --log-file=/destination/dir/rsync_log.txt -e ssh remoteuser@remotehost:/source/dir /destination/dir/

Here's what the options do:
-a archive
-v verbose
-z compress
-u update  <= Do Not Overwrite the Modified Files at the Destination
-h human-readable
-P progress bar/partial transfers

Note that these options are turned on in '-a' archive mode:
        -r, --recursive             recurse into directories
        -l, --links                 copy symlinks as symlinks
        -p, --perms                 preserve permissions
        -t, --times                 preserve times
        -g, --group                 preserve group
        -o, --owner                 preserve owner (super-user only)
        -D                          same as --devices --specials