SSH
I use it so many times and it also widely used among programmer and sysadmin, cool software that make you easily connect to another machine, securely and remotely!
I use the openssh implementation, it support some sort of ftp (SFTP) as well, using scp
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The other feature which I used so many times is forwarding tcp packet (it also support udp), for example to access a service from internal or private network, also useful for remote display such as forwarding X11 protocol or using VNC protocol.
hidden easter egg I found on serverfault, you can now exit from broken ssh connection, cool! (you might want to consider set keep alive option to avoid crash or broken connection)
other use cases is to authenticate git, which I already cover the topic on separate post.