There are a ton of XDA forums and a decent number of answers here that refer to specific dialer codes. For instance
*#*#4636#*#* will cause unicorns to spring forth from your phone or tell you what OS you're running and how much RAM you have (I forget which). I found one list of Secret Star Codes but I'm wondering if there's a more complete list? Someone warned that some codes can really screw up your phone and it'd be nice to have a good place to point people for good information on what the codes actually mean.
Though in fact
*#*#4636#*#*, which was recently recommended, is fairly safe, I think it is a good practice, especially since we all start talking about fire breathing dragons when the subject of rooting a device comes up, to make sure we're encouraging folks to make sure they know what a dialer code is going to do.