Indeed, Wunderground's willingness to frequently add new stuff shows their responsiveness to the consumer. I've been a paying customer for years--since the first year they offered "No ads." It's well worth the annual fee.
I have never paid, I am not sure why. I signed up as a photographer the day it hit their site and it said free services came with it for a year... that year passed and I never paid a dime and keep getting the services. Not bad! ;-)
Even their free offerings are pretty cool, and I am sure they will continue to grow quite a bit in the coming years.