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SSH-2 is a much more flexible protocol than SSH-1. It doesn't require exactly one terminal session per SSH connection; you can have more than one, or indeed none at all.
It would be nice if PuTTY could make use of this. For example, there could be a System menu option to start an extra terminal window inside the same SSH connection as the existing one(s). This would be more efficient than the existing Duplicate Session command, and also wouldn't require repeated authentication (which you don't care that much about if you're using public keys, but you might care a lot if you don't want to have to type your password in again).
Going in the other direction, several people have asked if it could be possible to start PuTTY without starting a terminal window and a shell - just have PuTTY sit in the background and do port forwarding only. This would also be possible in theory.
Similarly, perhaps we should also support running (e.g.) psftp sessions over an existing connection rather than forcing a new TCP/SSH connection - similar to the "connection multiplexing" feature in recent versions of OpenSSH - using the same sort of inter-process communication as with Pageant. Needs careful implementation to avoid opening security holes.
Would we want the server part of such a feature implemented in all the PuTTY utilities, or only in a dedicated application, perhaps one created for no-terminal-window?
SGT, 2004-10-13: Just checked in the easy bit. PuTTY and
Plink can now avoid running a command (equivalent to OpenSSH's
-N option, and indeed enabled by the same switch).
(PuTTY will still put up a terminal window.)
SGT, 2013-11-17: now implemented a 'connection sharing' system configurable from the SSH panel. Instances of PuTTY can be told to share each other's connections where possible, in which case the first PuTTY to connect to a given user@host will set itself up as an 'upstream' able to forward stuff to the server on behalf of other processes, and subsequent PuTTYs (and PSCPs, PSFTPs etc) will then use that upstream if they can rather than make their own SSH connections.