American Impressionist Society
Nationally recognized as the best impressionists in America,  has an annual show and members from accross the country show every year. 

Catalina Artist Association
An excellent grass roots organization that sponsor annual art shows, workshops, and the casino art gallery. Members get to hang art in the gallery and have other benefits. 

Frank Ordaz
This excellent artist has a variety of artwork and his blog has daily updates and teaching tips. He has done a family protrait of the President family on the front lawn playing.  Interesting work and good teaching advice. 

Laguna Plein Air Painters Association
This organization is located in the art hub of southern california and educates the public on the benefits of this art form.  Offers events and learning opportunities by noted master artists.  One of the best associations around.

Plein Air Painters of America
This granddaddy of plein air association has a site just rennovated containing some  of the best painters in America. Check out their definition and each artist has an intro with their particular slant on the subject.