Sources for US Zip Code Boundaries?
Zip Codes are not polygons, and therefore do not have boundaries. Zip Codes are lines (delivery routes) or points (Post Offices). See the FAQ on the old Census Bureau website and the paper Zip Codes are Not Polygons for details.
That being said, when I need to use Zip Codes the first place I look is at the ESRI data that used to come on CDs with ArcView/GIS, but is now available online at Esri Data & Maps.
Maponics provides ZIP Code Boundaries and updates them each month with data directly from the USPS (and in fact, we are the only ZIP Code data provider that the USPS refers its customer to). While it is true that the data from the USPS is not in the form of geographic boundaries, we use numerous methods and algorithms to create boundaries from carrier routes and other data. The results are the most current available and can be purchased or licensed (with monthly or quarterly updates included) in a variety of GIS formats.
Update: Maponics is now part of Pitney Bowes and the zip code product is available at Zip-Codes.com.
A five-digit ZIP code covers an arbitrary collection of delivery points (addresses). The USPS is the sole determiner of what ZIP code is assigned to what address, so they are the best source of current and correct data, which changes monthly. Cheap sources of current data are vendors like Semaphore Corporation.
Addresses within a ZIP code are assigned to numbered delivery routes. Addresses along the route are visited in a specific order, however there is no guarantee what path is taken by the mail carrier between two points along the route, so attempts at plotting routes as lines (or plotting all routes in a ZIP as a collection of lines) can't always be accurate.
Because ZIP codes are collections of points, the concept of "boundary" is ambiguous. It might mean computing the minimum rectangular bounding box enclosing all the addresses in a ZIP, or using a Graham scan to compute a "tighter" polygon, or using Lloyd's algorithm to polygonize a map, but how to choose the "best" boundary isn't always clear as shown in these 3 different boundaries for 5 points.
Since all addresses for a ZIP will be served by a single post office, it's tempting to start with that office's city boundary as the encompassing border for all ZIP codes served by that office, but exceptions occur: for example, a post office will serve ZIP codes outside the city, such as in unincorporated towns. Small towns often don't have street delivery; everything for a ZIP code might go to post office boxes at one location. And just over 100 "unique" ZIP codes are reserved for schools and businesses at a single location.