Trivial SSH tunnel
ssh -l routingUsername -L localhostPort:destinationServer:destinationPort -N routingServer &
Note that routingServer can be the same as destinationServer but is required! -N means don't open a shell, just build the tunnel, and the & is to background it as it naturally stays in the foreground.
eg, if your ISP is blocking port 25 outgoing, use:
ssh -l myusername -L 2525:my.remoteserver.com:25 -N my.remoteserver.com
and connect to port 25 locally.