I've never found a frost-tolerant tomato (although you might evaluate 0-33 and Wheatley's Frost Resistant to see what you think), but I have found a frost and freeze tolerant member of the Solanaceae family (and the Solanum genus, no less). That is, Morelle De Balbis. Last year, my Morelle De Balbis plants survived until it hit 15° F. (long after all the other plants died). The fruit froze a few degrees above 15° F., however, but the plant survived until then. I saved seeds from both frozen fruit and unfrozen fruit. The unfrozen fruit's seeds germinated much faster (and they probably had a higher germination rate, but I'm not sure). The seeds from frozen fruits took longer to germinate, but they had all-around stronger and faster-growing plants (which are still larger at maturity, it seems). Both sets of seeds came from fruits harvested in freezing temperatures, however.

I'm guessing the hype about Coldset was about the seeds being tolerant to 18 degrees (not the plants). At least, Reimerseeds mentions the seeds rather than the plants. This isn't a remarkable claim, though. Lots of tomato seeds will overwinter temperatures much colder than that.

I've found eggplant to be more frost-tolerant than tomatoes, personally (but only slightly).

Frost in one area may be different from frost in another area, though.

I have had some of the thorny Asian eggplants and decided that I didn't care for any use I could think of for them ... Maybe, Morellel De Balbis is similar. This year, I tried growing Chinese Lanterns as an ornamental. Wow! If some in the family can take a frost, Chinese Lanterns proved that they wanted only the most pleasant conditions and my June garden didn't provide it. I'm not sure if they will ever bloom. They barely survived the seesaw temperatures in the early season.

Anyway, here is an article in Mother Earth News about the Marellel De Balbis. They say, "will take light frosts to as low as 25 degrees Fahrenheit without much damage" so there's some confirmation on frost hardiness.

I wonder if I'd had that inadvertent test of tomato cold tolerance by 2010 when I wrote the above post ... those were plant starts and I moved flats of tomatoes out of my heated greenhouse into an unheard plastic hoop house just in time for a light frost. Those tomatoes had never experienced anything below 60°f.

Those wonderful Bloody Butchers, which can produce so early, died! It was only 37° ! I know it was that temperature that killed them because they were sitting right beside the thermometer.

Here's the strange thing: the Bloody Butchers were in the middle of the hoop house. Some tomato plants were much closer to the plastic film. Branches died, leaves died ... no other plants died amongst the others! So, later maturing varieties survived the near-frost conditions better.

I really think that setting fruit is what is often being taken into account when accounting for early maturing. It doesn't necessarily have anything to do with hardiness ... maybe in a few more years of plant breeding, it will.

Steve

I'm growing Chinese Lantern gigantea, this year (seeds from Baker Creek; that's where I got my Morelle De Balbis, too). I must say that it certainly is a lot slower to mature than my other ground cherries. I'm guessing it takes off on its second year and after acclimatizing through seed-saving (wild versions of it are apparently hard to get rid of around here). My plants are kind of smothered by other stuff, though; so hopefully they'll survive. I might have to plant them again. I started mine in an unheated greenhouse and transplanted them in May, I think. They grew and survived fine (just very slowly). They were a lot easier to germinate in the greenhouse than indoors, I thought, however (same with Morelle De Balbis). Giant Cape Gooseberry is also easier to start in a greenhouse, I think.

About the taste and use of Morelle De Balbis, it can potentially taste totally awesome, but I think you have to let it get super super ripe, first (like longer than after the time they fall off the plant). I had one or two that tasted like awesomely sweet strawberries, last fall. The ones I've tasted this year, so far, taste kind of like people usually describe them (tomato mixed with cherry, sort of). Maybe they need to be frozen or cold-ripened to get the awesome strawberry taste. I'm sure one could find ways to use them. I should note that my seeds were harder on the first year. This year, they're more chewable, like tomatillo seeds (and not unpleasant).

I can't comment on "cold set", but Joseph Lofthouse of Utah is currently breeding for cold tolerant (and by extension frost) tomatoes. It is still an early breeding project, but his early results are encouraging. He's already had like a 90% die off rate, but the genetic variants that remain continue to get more cold and frost tolerant season by season.

http://alanbishop.proboards.com/thread/6988/tolerant-tomatoes-right-josephs-alley?page=19

Joseph Lofthouse is awesome. He breeds landraces and stuff. I didn't know he was working on frost tolerance, though. I know some other people are, however. A lot of the varieties might only be available in Canada, currently, though; so, they would require a Small Lots of Seed permit to legally import them into the USA. However, apparently, Small Lots of Seed permits are free (so don't hesitate to spending some time to find out how to get one). There are two methods to get one (one, you just send in a form or something—although it may be difficult to locate the form, and the other, you have to go to some place, register, and then you can do it online). See this link from a Canadian store (which might not be obviously Canadian, if you're not careful), for more information: http://tatianastomatobase.com/wiki/Category:Seed_Order#tab=Terms_of_Service

Oh, and since I talked about Morelle De Balbis, know that it seems to taste significantly sweeter when the weather has been cold and frosting for a few weeks or longer. I noticed that this year. It wasn't that sweet this summer.