Illuminating sky option won in tutorial poll, so here it is. It provides basic guide on how to tune up stock into moody and diffused sky.
Software: Adobe Photoshop CS5.
Newbie alert: Photoshop =/= Photoshop Elements
Level - Easy (no clippings, no masks, nothing particularly time consuming)
Effect values may vary depending on your stock image size.
Other than that, have fun!
More tutorials here: [link]
Oh and I totally mean business in the end