Consult the "Ark of Taste" for New England from the US Slow Food folks. Plant some of the varieties "boarded" on the Ark to help save a New England agricultural treasure from extinction. http://bit.ly/fbZps8
Plant some RAFT seeds. Renewing America's Food Traditions (RAFT) of the Chef's Collaborative (located in Boston) conducted an annual "grow-out" of around 10 fabulous-but-threatened heirloom veggies. Try some of last year's varieties (no grow-out this year): http://bit.ly/dA1bCz
Look slightly south. With global warming, plants have to deal with higher temps and longer seasons. Try heirlooms with a long history in, say, the mid-Atlantic states. Also among similar climates across the world.
Take some chances and try seeds you've never grown (or eaten, or maybe even heard of!) before. They could become future favorites!
Buy from seed companies doing great work. There are some wonderful companies working to save our food and agricultural heritage -- support them! Here are three in our region that grow their own seeds, and focus on preserving heritage seed stock: Fedco, and the just revitalized Comstock, Ferre & Co. Also Seed Savers Exchange does amazing seed saving work world wide - become a member and get seed through their mind-blowing "yearbook".