Fedco Seeds and Johnny's Select Seeds are our two Maine favorites; other excellent North American companies are listed in this article in Mother Earth News: http://www.motherearthnews.com/Organic-Gardening/2007-11-01/Best-Garden-Seed-Companies.aspx
I'm also tempted by:
http://www.chilternseeds.co.uk/ & http://www.mr-fothergills.co.uk/