CIMM  True, it's not the most accurate database there is but it's free an has a very liberal license. Have a look at www.geonames.org, it's a wiki like system so you can improve the database yourself. This will not only improve ZipCarton but all other services that use GeoNames as well.

It's an open source application with a public domain database as source. That's what I like about it: free and open. If everyone helps a very tiny little bit to improve the database we all get something out of it.  
(Version 0.9)

CIMM  There was a bug in the MacUpdate website that showed 14MB in stead of 14KB which explains why it seemed huge, MacUpdate solved the problem.

Thank you all for the kind replies. I created ZipCarton because it annoyed me to go to Google each time to find the ZIP code and maybe it can help others as well. If you don't like it don't download it or even better, get the source code at http://svn.milkcarton.be/zipcarton and improve it.  
(Version 0.9)

CIMM  Thank you for the feedback. The API looks interesting but I'll have to check if we can apply for a "special permission" as they say on the website. I didn't walk this way yet as it means we need to include different logic for each country and check which countries provide these services (I contacted the Belgian postal service for example but they didn't even reply). So, in short, yes, you are right, it would be better and more accurate but I can't bare the workload at the moment to maintain it.  
(Version 0.9)