You probably didn't even know there was a PFRE sky library did you. Yes, since March of 2006 there has been one. Many readers over the years have donated public domain skies for everyone to use. It now has 133 sky photos.
The library hasn't been updated for about a year but a couple of days ago Iran Watson donated 10 of his favorite skies to the library so I've just updated the library to add Iran's skies. Thanks Iran!
To access the library browse through the library as a slide show, decide which sky you want, then click on the thumbnail to display the full-size photo and then right click and select "Save Image as..." to download the sky.
For those that need help using these skies to replace the sky in your photo see the past tutorials on PFRE on the subject of sky replacement. Here is a list of some of the video tutorials we featured in the past:
Oh yeah, The permanent link to the new sky library is on the right side-bar under "Other Links".