I was pleasantly surprised to learn that OpenSSH has a number of very useful commands available via an escape sequence (the default escape key is ~ preceded by a newline, and can be configured with the EscapeChar directive in your ssh_config). The ~? sequence presents a list of available commands:

Supported escape sequences:
  ~.  - terminate connection (and any multiplexed sessions)
  ~B  - send a BREAK to the remote system
  ~C  - open a command line
  ~R  - Request rekey (SSH protocol 2 only)
  ~^Z - suspend ssh
  ~#  - list forwarded connections
  ~&  - background ssh (when waiting for connections to terminate)
  ~?  - this message
  ~~  - send the escape character by typing it twice

(Note that escapes are only recognized immediately after newline.) The ~. sequence is worth committing to memory for those (frequent, for me) situations where you wind up with an unresponsive ssh session. The ~C option is also handy, in that allows the opening/closing of forwarded ports on-the-fly:

      -L[bind_address:]port:host:hostport    Request local forward
      -R[bind_address:]port:host:hostport    Request remote forward
      -D[bind_address:]port                  Request dynamic forward
      -KR[bind_address:]port                 Cancel remote forward

Very useful to know if you use ssh frequently.