"Enter" then ~ then . will kill a hung SSH connection, instead of having to close the terminal tab. I use it all the time but most people haven't heard of it.
To be more precise ~ is the default ssh escape character. It is only treated as the ssh escape character if it is at the beginning of a new line. From the ssh(1):
> The escape character followed by a dot (`.') closes the connection; followed by control-Z suspends the connection; and followed by itself sends the escape character once.
There's more commands than the dot too but I admit I forgot the others
~?
will print a list of the available escape sequences.
EDIT: I started distrusting my memory here and went hunting, it appears my memory has the escape nesting backwards https://lgfang.github.io/computer/2009/05/08/quit-nested-ssh...
It was [wait]+++[wait] and then ATH or ATH0 by the way, on a modem that implemented the Hayes command set correctly. To avoid at least some inadvertent triggering of this in-band signalling.
> is this the same as hitting CTRL-D?
No. ^d is a character that is sent to the remote shell (bash, zsh, etc) to tell it to exit. ~. is something for the SSH client to tell it to cut the connect if (e.g.) the shell has hung.
Thing of it like working at different OSI layers: ^d is HTTP, while ~. is like TCP or IP.