I've used seeds from William Dam Seeds for several years now. Last year most of my garden was grown with seeds from William Dam that I'd had for 3 years and that hadn't been stored entirely properly. I started several hundred plants and the germination rate was still over 90%. I wasn't surprised as I've been very happy with their seeds for years.
They sell in small packages, but also provide larger packages at better prices. They do a lot of mail-order business, though I've always gone to the store. They've always been helpful and I've found their prices to be better than other stores, especially for trays, fabric, plastic, etc.
They are also recommended by Steve Solomon in his book Gardening When it Counts and they are the seed company that I judge all others by. I give them a 9.5.
A piece of land is worth as much as the person farming it.
-Le Livre du Colon, 1902