Tested Google Nexus S Secret Codes

Secret Code for i9020/i9023/M200

*#*#786#*#*
RTO = Restore To Original
Hard Reset (Wipes/Resets Data/Profile & internal storage)
*#*#4772579#*#*
GPSCLRX
GPS clear (reset)
*#*#4636#*#*
INFO
Testing menu (Phone, Battery & Wifi Info, Usage Statistics)
*#*#8255#*#*
TALK
GTalk Monitoring (Monitor GTalk Activity)
*#06#
IMEI number
*#*#2432546#*#*
CHECKIN
Checkin (Manually check for System Updates)

*#*#3282#*#*
Data Programming (Data Profile, DDTM, Roaming Orig. Vocoder, EVRC-B, Home SID, USB Charging, RTSP/HTTPPD ports & addresses, Browser Gateway, MMSC URL)
*#*#33284#*#*
Debug Menu (1x/EVDO Engineering, , Data Status,Debug Screen, Test Calls, RC Data, HDR/1x Selection, Voice Privacy, DTMF Set, Korea Mode Set)
*#*#786#*#*
RTO = Restore To Original
Hard Reset (Wipes/Resets Data/Profile & internal storage!)
*#*#(your msl)#*#*
Service Menu (Phone Number Change)
*#*#8778#*#*
Phone Utility (UART/USB Modem & PDA Selection)
*#*#4772579#*#*
GPSCLRX
GPS clear (reset)
*#*#4636#*#*
INFO
Testing menu (Phone, Battery & Wifi Info, Usage Statistics)
*#*#8255#*#*
TALK
GTalk Monitoring (Monitor GTalk Activity)
*#06#
MEID number (Display your MEID/IMEI)
*228
Phone activation (Activate your phone)
*#*#2432546#*#*
CHECKIN
Checkin (Manually check for System Updates)
*#*#72786#*#*
SCRTN
Factory default value for DSA selection