So I'm in the process of reading through Matt Berseth's blog after the last post with his awesome GridViews and found another really neat item he's done. By using ASP.Net Ajax and the Virtual Earth Map Control, Matt has created an application that locates a zip code's boundary. He then has a follow up post on how he got the data for doing such a query to figure out the boundaries on that zip code. Matt was even nice enough to have the data so you can import it in to your own local SQL Server.
Here is the link to play with his demo on his server.