And I really mean official, you don't need to throw in your action replay/xploder/whatever!
So, for the serial port, I can only think of:
-the sony link cable
-sony net yaroze serial cable
-koei parallel printer adapter (with and without video capture)
Now more interesting, for the "parallel" expansion port, there is:
-the psy-q blue dev cart, similar to the one for the sega saturn
And another device, with very few informations, in fact I'm not even sure how it is connected to the playstation:
-the dtl-h1500 network adapter
Do I miss anything?