Here's a list of some plants that will grow in Zone 6.

Abelia (Abelia x grandiflora)
Japanese Anemone (Anemone x "Prince Heinrich")
Rhinland Astilbe (Astilbe x 'Rheinland')
Candytuft (Iberis sempervirens)
Florist's Chrysanthemum (Chrysanthemum x morifolium)
Martha Washington Geranium (Pelargonium peltatum)