After going through many CSS galleries, I observed a common feature - clouds and the beautiful sky. It simply adds gradients and informal, illustrative and vast look to the headers. Some of the designs even went into details like mountains and flock of sheep in it.

But sometimes, the same objects; Sky and clouds have been used in a subtle way. See those effects inHopking Designs and Land Experts. Enough talk; See them.

#11) You Rant

#10) Hopking Design

#9) Land Experts

#8) El Pescadito

#7) Welika Villege

#6) Bin Slash Bash

#5) Global Zoo

#4) Joshua Brayant

#3) WiredTree Hosting

#2) Arty Papers

#1) UX Magazine