scp is probably the safest, and easiest without installing ftp daemons.
Single file:
scp file.ext user@server2:/location/to/put/file
ex: scp index.html pun@server2:/home/pun/public_html
Directory:
scp -r directory_name user@server2:/location/to/put/directory
ex: scp -r images pun@server2:/home/pun/public_html/
If you have an ftp daemon already installed on both servers, then you can obviously use that. FTP is faster than scp due to the lack of encryption, but everything is sent plain text, which may or may not be in your best interest.
(Note: This is all assuming you have shell access to some sort of unix based machine.)