Here are a few blogs and web pages I have found about Plein Air Painting:
Raynald Murphy plein air tips in English and in French
Sarkis Antikajian's webpage
Bets Cole's webpage
Plein Air Painters of American (well at least Catalina Island)
International Plein Air Painters (of Niagra Falls)
Plein Air Painters of Oregon (actually of Bend)
What's in a color?
This is more that you ever wanted to know about artist's paints and what is in them.
It describes how paint manufactures describe light fastness and how you can determine exactly what is in the paints you are using. Amazing information.