I saw one of these for sale in a gardening catalog and did a double-take. Ripening green tomatoes, or most other green fruits and vegetables, is not hard to do, and you certainly don't have to buy an expensive gadget to do it.
Easy to Ripen At Home
I use a brown bag inside a copper pot so that the bag can be stored on my counter. The key is adding a fruit - either an apple or banana, and sealing the brown bag so the natural gas ethylene, which fruits generate, will help ripen whatever is still green. I use this ripener to help mangos, guavas, tomatoes, and sometimes bananas and grapes get ripe.
Check once a week or so. Smaller fruits and vegetables like tomatoes may ripen faster than larger ones. Remove any tomatoes that are getting dry; others may start going brown. Also periodically replace the fruit - an apple may last for a couple of months before it needs to be replaced. Apples last longer than bananas in brown bags.
Remind Yourself to Check
I top the copper pot with a little glass plate and use it as a fruit plate. It's purely decorative but it is a good reminder to periodically check the brown bag. You can also just keep your green tomatoes, or other produce, in a brown bag in a dark cabinet corner. Tie a ribbon to the door knob to remind yourself to check it once a week or you won't want to try to ripen anything again.
How do you ripen your fruits and tomatoes?