Postcodes On-line 
The Royal Mail's online postcode service. Look up a postcode for any address in the UK. Displays the address in full as a mailing label that can be cut & pasted into any Windows application. Once you have the postcode, you can also view a detailed location map if you visit Multimap.com
Postal Codes & Postal Sites Worldwide
Not an attractive site to use, but if you persevere and keep scrolling down, you will find links to postal codes and Post Office sites all round the world.
Postal Rates
A very useful site providing links to postal sites world-wide giving postal rates, postcodes & postal authorities.