As Bamboo said, Litchi tomatoes are not tomatoes at all, although they, much like eggplant and wild tomatoes, are in the same genus (Solanum). It might be better if we call them Morelle De Balbis or Fire and Ice Plants (but that doesn't tell us what to call the fruit).
I have heard several anecdotal accounts/claims of frost-tolerant varieties. However, none of them have been proven to the world, yet, and I've never grown a tomato that got anywhere near Morelle De Balbis for frost/freeze tolerance, personally. I am the one who grew Morelle De Balbis and reported its freeze and frost tolerance, though. It's a very interesting species.
I'm growing Morelle De Balbis again this year, and the plants are doing much better than last year. They now set fruit much earlier and in the heat (unlike last year), and they have bigger fruit (the largest are about the size of a good sized salad cherry tomato; the largest were about half that size last year). The earliness and heat-tolerance could be differences in soil, light and watering, but I like to think acclimatization has a lot to do with it.
From what I've read, tomatoes are more likely to tolerate light spring frosts than fall frosts, but I wouldn't count on either. I've also read that the reverse is true for a certain variety I'll list below.
Here is a list of a few of the tomatoes that some people think have frost tolerance (although one or more of them may have been disproved):
- 0-33 (This is probably the one most likely to be true)
- Wheatley's Frost Resistant (it's in the name)
- Anna Russian is said to tolerate mild frosts
- Sunset's Red Horizon Huge (I don't know if this is the same as Sunset's Red Horizon, but Sunset's Red Horizon is supposed to be a good tomato.)
- Sunset's Red Horizon
- Coldset (I believe this has been disproven, or at least claimed against. I get the feeling it's just the seeds that have been shown to be freeze-tolerant to 18° F. rather than the plants.)
- Sweet 100 (according to one person, but not another)
I personally think it's possible to acclimate tomatoes to tolerate frosts (through generations), but I don't know of any frost-tolerant tomatoes that exist today with any amount of surety.
It should be noted that I've heard people talk about frost-tolerance in peppers, too, with some varieties/species, such as Rocoto and Goat's Weed. I've grown Rocoto and Black Cobra (which is similar to goat's weed), and I haven't found them to be particularly more frost-tolerant than other peppers. I think it's possible that there are different kinds of frosts, and they may resist some kinds but not others. Or, the frosts could have just missed those varieties (since it doesn't always kill things equally). It's also possible that my plants weren't mature enough to be frost-tolerant. Maturity seems to matter for some perennials to handle freezes, at least.
The Morelle De Balbis (all the plants I planted) definitely survived a lot of hard freezes, though (and I think it might have even snowed on them). But they did die when it hit 15° F. The fruit wasn't as freeze-tolerant as the plants, but the fruits were still freeze-tolerant (maybe to like 17° F. or something; the fruits didn't freeze solid until then).
I'm curious to see if my Morelle De Balbis plants that came from seeds of frozen-solid fruits will resist temperatures any colder than 15° F. this year.