Topographically Integrated Geographic Encoding and Referencing (TIGER) is a series of spatial data releases managed by the US Census Bureau. Many of its smaller area geographic divisions known as Census tracts and blocks are used as a standard in spatial analyses.
TIGER has many types of spatial datasets that are released at least once a year, including:
Metropolitan Statistical Areas
Places (Aka cities/unincorporated areas)
ZIP Codes (through the edges shapefiles)
Zip Code Tabulation Areas (ZCTA) - Areas dominated by one particular zip code School Districts Voting Districts Traffic Analysis Zones
The information is avaialable in an ftp server.