Get your own Wundergound API key
Sign up is easy. First you'll have to register as a user for Wunderground. The second step is to obtain an API key. Both steps are covered here.
Note: you will have to "purchase" a key, but the cost is $0! Yes, it is free.
1: Go to the Wunderground registration page and enter your personal details
1c: Press 'Become a Member' and confirm your e-mail address
You're now about to receive an e-mail with a confirmation link. Simple activate the liink to validate your e-mail address.
OK, so much for part 1. Are you ready for the next part? It is equally difficult....
2a: Go to the 'Get API key page'
2b: Hit the 'Explore My Options' button
2c: Select a Developer license for the 'Anvil Plan' and purchase the key
OK, Wunderground will charge you nothing for this service. Great 'Value for Money'!