Rewriting (proxying) a TCP stream in one line

Damn unix is great...

Step 1: In a shell, set up netcat to listen on TCP 5500:
nc -l -p 5500

Step 2: In another shell/on an intermediate machine..
nc -l -p 50000 -c 'sed -u -e s/billy/freddy/g | nc localhost 5500'

Step 3: In a third shell,
nc localhost 50000

..and start typing stuff, including the word "billy"

Ok so the third shell will talk to the first shell and replace "billy" with "freddy".
Note this is bidirectional - type in first shell and it goes back to third one (without rewrite)

How handy is that? VERY!

If you need a TCP stream (e.g. gnu debugger) intercepted and rewritten slightly.. that's yer man.

Thank you beardy unix gods from 1972!