Free service for maps from Yahoo!  - http://maps.yahoo.com/  

Free service from MapQuest! - http://www.mapquest.com/ 

Map demographics, etc. by zip code. - http://library.csun.edu/mfinley/zipstats.html 

Not a map site but sure useful. - http://www.usps.gov/ncsc/lookups/lookup_zip+4.html