My own recent interest in open pollinated varieties, some heirloom,
have led me to Seed Savers Exchange:
http://seedsavers.org/Home.asp

These guys currently get the lions share of business from me, just this
year I purchased a membership. I believe their work is important, in
preserving heirlooms, maintaning a vigil against genetic modification.

I've purchased from Johnnys Select Seeds, lots of variety. Prices seem
to have gone up faster than inflation recently..

Burpee occaissionally.

Jungs because they seem to have consistantly lower costs - but
sometimes you get what you pay for.

Thompson & Morgan, they sometimes have varieties not available
elsewhere.

Gurneys, that discount can be pretty good if they carry something you
wanted to get anyways, like fruit trees or grapes..

How about yourself?