There are several reasons why you might be curious about what to do with an old hearing aid. You might have just updated your old one with a better unit, you might have received a cochlear implant and thus have no need for yours any longer, or it might have belonged to a relative who no longer needs it.
If the hearing aid was yours, the first option to consider it to keep it as a back up or a spare. If you lose or damage your new hearing aid and are without it for a period of time, you can revert back to your old one temporarily. This idea has some merit, in the same way that keeping your old pair of glasses when you get a new pair is a smart idea, just in case the new pair gets lost or sat upon.
You could consider selling the hearing aid on Craig’s List or eBay, especially if it was originally expensive and it is still in good working order. Before you do this, make sure it is legal in your area to resell hearing aids; there are many states in which it is not. When creating your ad, list all of the information you would want to know if you were buying a used hearing aid: the model and manufacturer of the unit, its condition and age, whether it is still under warranty, and why you are selling it. Also, don’t expect to get nearly as much money as you paid for it; look for ads for similar hearing aids, and keep yours within the same range.
However, one option that is available to you if you can afford it is to not sell or keep the hearing aid but to give it away to someone who is in need of one. Find an organization that collects used hearing aids, refurbishes them (if needed) and distributes them to new owners based on need. In the United States, organizations that specialize in the charity collection and redistribution of hearing aids include the Lions Clubs International, Hear Now by the Starkey Foundation, and Sertoma. Another organization called Worldwide Hearing also collects used hearing aids for redistribution in developing nations where the people need them but cannot afford them.