For heirloom and unusual seeds:
www.rareseeds.com - Baker Creek Heirloom Seeds
www.comstockferre.com - Comstock
ebay sellers (another39again, rsedwards64, dragonfly183, frankyflynn, everwildeflowers, grocoseeds)
AND MORE! LOL But, I use these the most, the ebay sellers are VERY inexpensive, especially on shipping, items come in clear ziplocs with labels, and sometimes planting instructions.
I also bought from Baker Creek (rareseeds.com), Comstock Ferre and Seed Savers. Pics and descriptions are very important to me and I'll be taking photos to send to these folks for their use in catalogs. I'll also be posting to my blog adn photobucket site. I've never heard of everwilde.com and plan to visit as soon as I post this.
Sorry to enter so late into this conversation, but i'd like to add that we always use FedCo as our primary source. I haven't compared prices across many sites, but Fedco is much cheaper than Johnny's. I do use Johnny's for things i can't get from Fedco, and Johnny's web sit has a lot of good tools, including one that tells you when to start your seeds indoors. You just enter the date of the last frost (i always use May 15, though i've never seen a frost at my house that late since i moved here from Colorado 8 years ago).
Another nice thing about Fedco is that they never use seeds from any source owned by Monsanto (such as Seminis, which Johnny's does carry).