Map helps you see neighoring zip codes - Maps.Huge.Info has long had a great list of Google Maps based boundary and zip code tools. Geo Developer John Coryat has informed me that he has improved his zip code map to use a custom tile layer which covers the US from zoom level 5 to 17 with potentially millions of tiles. When you search a zip code you see all other zip codes close by - give it a try!
County and Zip code Boundary Maps - Here is another zip code mashup of census.gov data and the Google Maps API (V.2.57) that lets you enter a zip code and plot the county lines that zip belongs to and also the zip code boundary. There is no restriction on the number of hits/second on this so have at it. Also, the polyline XML files are available for download for any given zip code.