I now have a HTC Desire smartphone running Android 2.2 (Froyo). I have confirmed that the following “secret” codes (aka. “easter eggs”) works on my device. They are supposed to work on other Android devices (e.g. Samsung Galaxy) as well.
*#*#4636#*#*(or*#*#info#*#*)- Testing – gives access to five different information screens.
*#*#8255#*#*- GTalk service monitor – monitors Google Talk host address & port, your Google JID, your Device ID, GTalk connection status, your connection history, your current GTalk presence (available, idle or invisible), etc.
*#*#7262626#*#*- Fieldtest – pressing menu lets you do setup, or select item to test.