I've been a fan of green tomatoes for years but they never seemed to sell as well as red tomato plants. There is a misconception that they are hard and flavorless but far from it, true green tomatoes that hold their green colour when ripe are "melt in the mouth", with a complex flavour like walnuts.
If you haven't grown a true green tomato, then Aunt Ruby's German Green is an excellent one to try. It comes from a lady by the name of Ruby Arnold who grew it in Greeneville, Tennesse for many years. It is now widely grown by heirloom gardeners in America and has a sweet spicy flavor. The fruit is large and excellent as a slicing tomato.