National

• Plein Air Painters of America (PAPA)

North East / Mid-Atlantic societies of note:

• CT: Connecticut Plein Air Painters Society (CPAPS)
• Mid-Atlantic: Mid-Atlantic Plein Air Painters (MAPAPA)

New Jersey

• Highlands: Plein Air Painters of the NJ Highlands
• North Central: New Jersey Society of En Plein Air Painters
• Lambertville: River Rats (Artsbridge) Plein Air Group

Other Plein Air Societies of note:

• CO: Plein Air Artists of Colorado (PAAC)
• Laguana, CA: Laguna Plein Aire Painters Association

Other Organizations featuring plein air workshops and paint outs

• CA: California Art Club
• National: Oil Painters of America

Other Resources

• New York Plein Air Painters (NYPAP) has a fairly comprehensive list of plein air societies.
• Plein Air Magazine and its website
• Wet Canvas often has postings on plein air gear and techniques
• PleinAirArt.Net (PAA) Listings of plein air events, workshops, and artist shows