Here's the list!   
 
Baker Creek 
I love these people - I've been getting Jere Gettle's catalog since I think he was still in school, and he sure has turned his passion into an amazing company! The catalog is absolutely gorgeous - the photos make it clear that these people sincerely and truly love vegetables and plants! And the variety of totally cool things! Just awesome!
 
Bountiful Gardens
This is John Jeavons' company. He is the pioneer of Bio Intensive Agriculture, and has written How to Grow More Vegetables and Fruits (and Fruits, Nuts, Berries, Grains, and Other Crops) Than You Ever Thought Possible on Less Land Than You Can Imagine and The Sustainable Vegetable Garden: A Backyard Guide to Healthy Soil and Higher Yields, two of my favorite gardening books. I practice Bio Intensive Methods on my farm and have based much of my own gardening-philosophy upon his books. The seeds he sells are chosen and developed to work well in intensive, organic, sustainable systems.   


Horizon Herbs/Strictly Medicinals
Horizon Herbs sells only certified organic seed and plants. If you want to find a rare and unusual herb seed that is also organic, this is where you go! Owned by Richo Cech, who is one of the top North American Gurus of medicinal herb growing, this is a huge resource! 

Hudson Valley Seed Library  
This is a truly awesome place to get your seeds - you can join the Seed Library as a member. Members can save the seeds from the plants they grow and return them back to the Seed Library. You are a part of the whole process of building a Hudson Valley seed bank!      
 
Johnny's Seeds

Johnny's carries a large percentage of certified organic, no GMO, Safe-Seed Pledge.   
The Johnny's Seeds Catalog is jam-packed with info and direction and insight. It really is a great reference tool. This is where I buy the bulk of my vegetable and greens seeds.  I always get great germination, and the company seems to focus its inventory and development on productive, dependable seed.
 
Kitazawa Asian Seed Company 
A fabulous source of Asian seeds of all sorts. 

Peaceful Valley Farm Supply 
Peaceful Valley carries a selection of seed brands, but they also have their own line of certified organic seeds, which are well-priced and high quality. And they offer free seeds with purchases. I get all my bulk cover crop seeds from them as well. Their catalog really does contain everything anyone growing organically could use - fertilizers, greenhouse supplies, soil amendments, row cover, etc. etc. .... Check out their growing videos too - very informative! 

Seeds of Change 
A wonderful selection of all certified organic seeds.     

Seeds from Italy / Franchi
Franchi Seeds of Italy is the world's oldest family owned seed company founded in 1783.This is where I get my arugula, many chicories, punctarella, agretti, erba stella, white gherkins, escarole, wild fennel.

Stellar Seeds  
The seed assortment of varieties from Stellar is well-crafted - you can easily get all your garden seeds from here and have an extremely unusual and exciting mix of vegetables, herbs, flowers, and greens.
 
Seed Savers Exchange
Dedicated to heirlooms! A source of over 600 heirloom and open-pollinated varieties, and a large selection of organic seeds.

Turtle Tree Seeds 
Turtle Tree features unusual European varieties that are steeped in Biodynamic Tradition, as well as being certified biodynamic. I've always done great with these seeds. They even have Mangels, a traditional winter feed for rabbits and chickens. I also make sure to buy the 4 of the 5 most important biodynamic herb seeds from here - Stinging Nettles, White Yarrow, Valerian, and German Chamomile. (The 5th is Dandelion, actual dandelion, taraxacum, for which I don't need seeds!)


First, a quick explanation of seed labeling....
 
Certified Organic Seeds
Certified Organic are seeds that have been harvested from certified organic crops. These seeds may be a hybrid or heirloom variety, but they cannot be treated with any chemicals. And certified organic seed cannot be Genetically Modified (GMO).

Heirloom Seeds
Heirlooms are seeds from cultivars or varieties that were grown and used during earlier times. Basically, they are seeds that were pushed aside for modern, large-sized, factory-style agriculture methods. Heirloom seeds have been carefully saved and stored and preserved over many years by people who appreciated their unique characteristics. Many heirloom seed varieties have retained their traits through open pollination.

Open Pollinated Seed
Open pollination is pollination by natural forces like wind, insects, birds, etc. Open pollination is nature at work adapting and changing and diversifying. Open-pollinated plants will produce seeds for new generations; however, because the pollen of these plants is uncontrolled, open pollination may result in plants wide genetic variations. This increases biodiversity, although if you are not careful you may lose the characteristic you love about a certain heirloom. You can do careful breedings of open pollinated heirlooms using row cover to insure that your heirlooms stay true.

Hybrid Seed
A hybrid seed is a naturally-bred seed, although it is not open pollinated. If a seed is labeled as a hybrid, that doesn't mean that the seed is bad or factory ag or GMO. Hybrids are bred to improve the characteristics of the plants, creating varieties with more yield, uniformity, disease resistance, fun colors, and so forth. Hybrids are produced by manually cross-pollinating plants. Hybrid seed cannot be saved, as the seed from the first generation of hybrid plants does not reliably produce true copies. The parent plants of hybrid seeds were bred naturally - they were not genetically manipulated as GMO's are.

Sourcing Seeds for Your Organic Garden 

midsummer Farm

Seed-Sourcing - some of my favorite places to get great seeds:
I believe in diversity, and I am a bit obsessed with plants so I order seeds from a lot of different places. But I do have my favorite sources that I go back to year after year.

As a certified organic farmer, I must always order certified organic seed. However, the National Organic Program recognizes the need for diversity, and so if there is a variety of something that I can't find anywhere organically, I'm allowed to buy it, as long as it isn't genetically modified, pelleted, or treated. I also try to save seed from those plants for the next year, building a bigger base of organic seed diversity.

So the following list has a lot of certified organic seed companies as well other companies who sell heirloom and wonderful varieties. None of the companies below sell Genetically Modified (GMO) seeds - they have taken the "Safe Seed Pledge."