I am convinced that everything that could be done with the old (and now dangerously insecure) applications ftp, telnet, rlogin, and rsh can be done just as well or better using the (cryptographically secure) applications ssh (slogin) and scp. Why am I convinced? Because I've done it. Everything from routine logins, to file transfers, to cron jobs, to remote system management, I've done them using ssh.
Sure, I am a cryptogeek, but the tool set that comes with ssh makes it easy for anyone else to be cryptographically secure without much effort.
This is an index of the web pages full of tips and tricks that I've written up as I informed others how to do likewise.
The other version in wide use is OpenSSH which comes from the OpenBSD operating system. This version is based on an early revision of ssh -- before it became commercial. It now supports the same basic connection protocols as the Finnish ssh does.
Indeed, the login and remote shell access protocols are pretty much guaranteed to interoperate because they are described by relatively mature Internet Drafts published by an IETF Working Group on SECSH. But these two different ssh implementations do not interoperate in all fashions. In particular, the drafts regarding protocols for file transfer, agent authentication, and proxy forwarding of ports and X11 are much less mature. I have uninformed suppositions of why this is so. If these suppositions are more libelous than true, please let me know and I'll endeavor to remedy the defects.