The following plants are ones I discovered growing here at Mountain Gardens in the Appalachian Mountains of North Carolina at 1.000 m height in USDA hardiness zone 6b. Many of these plants are said in the literature to not be hardy enough or are normally associated with a warmer climate.
Fig (Ficus carica) is listed as being hardy in zones 6-10 but it can be hard to find a variety that really is hardy in zone 6 without dying back to the ground every year. This one didn't die back since it was planted.
Peach (Prunus persica) is normally associated with warm climates, but there are varieties that are very hardy. At PFAF it is even listed as being hardy in zones 5-9. Especially 'Siberian C' is very hardy, which this one might be. 'Siberian C' also is said to be true to seed (is self-fertile), so the kernels can be planted and the resulting plants should have similar properties.
Pomegranate (Punica granatum) is listed as being hardy in zones 8-12 according to PFAF. Again there are some varieties that are much hardier. This one is 'Russian'. It did die back this winter, but grew again from it's roots. If it gets to survive a few winters without dying back the wood may become strong enough to withstand the cold winter temperatures.
Tea (Camellia sinensis) USDA hardiness zones 7-9 according to PFAF. As I have written about in my post Green Tea in Zone 6b, there are many hardy tea varieites.
Jiaogulan (Gynostemma pentaphyllum) USDA hardiness zones 7-10 according to PFAF. Jiaogulan is often cited as not being hardy even in zone 7. Here it grows rather invasive, both the sweet and the bitter variety.
The most hardy? ginger species of which the flower buds are eaten, sometimes said to be hardy even in zone 5. Here it survives without an extra mulch protection.
Yacon (Polymnia sonchifolia, syn.: Smallanthus sonchifolia) hardiness zones 7-10 according to PFAF.
Taro (Colocasia esculenta) hardiness zones 9-11 according to PFAF. Survived only one winter yet and it may have been a rather mild one, still very surprising.
The following plants are hardy in the unheated greenhouses here: