There are only two well known Sony Ericsson secret codes which allow you to access any hidden menus on the device and from what I can tell most of the newer models no longer include a lot of the options from the older phones however if you have the modern access codes just send them to me in a mail and I will add them here.
The standard *#06# which displays the IMEI number and then a menu accesible via the directional pad or joystick, simply enter the following sequence on the main menu screen.
-> * <- <- * <- *
This will open a menu with a large variety of options based on your model of phone, some of the possible menus available here include;
Software Information, Configuration Information, Sim Lock Status, Total Call Time, Text Labels, Real time Clock
There is also a section for testing your phone hardware and features which once again depends on which Sony Ericsson phone you have but normally includes;
Main display, LED/illumination, Flash LED, Keyboard, Earphone, Speaker, Radio, Camera, Vibrator, Microphone
There are some shortcut codes which are not found in older manuals;
<- * * <- Queries the device lockstatus
0# Displays the call history tab
<-0000-> Resets the phone to the default language (normally English) or in some cases it takes you to the language selection menu which is good enough if you are trying to restore your phones language settings after a minor mistake or prank
Once again if you are aware of any other Sony Ericsson Secret codes please send me a mail so that I can add them to the list here.