I've been growing three different kinds of tomato plants from seed in my garage under grow lights. I put them outside on Saturday to get some real sun, and one of the plants broke off when a strong breeze came along. Bummer!
I picked off the broken stem and put it in a small glass of water on my window sill.
After just a few days, roots have started to grow. In a few more days, I'll plant it in a pot and when the weather warms up enough, I'll plant it in my garden.
This also works later in the summer when your tomato plants get leggy and stop producing. Just cut off some of the branch tips and root them in water for new tomato plants. Last year I still had tomatoes growing in October because I started some new plants in mid July.