This might be quite UK centric and is a very dry answer but old smartphones should be donated to asylum seekers.

I remember reading an article a year or so ago about how asylum live on the welfare afforded to them and their phone bills (a necessary expense) were a huge chunk of it, basically all of them paying well more than I do on my PAYG for similar service. What I see must be the issue is that, without a phone of their own, the cheapest short term option is to sign up for an option that provides a phone.

This immediately limits them from the ultra low budget GiffGaff/Asda/etc type options and will very frequently tie them down to a much less budget friendly carrier if not straight into a contract.

Old smartphones are good as security cameras: they have a nice camera and wi-fi of course, but also could be much better than traditional security cams as they also can have 3G/4G networking and a battery, in case power/internet is cut for example. They could also record or play sounds, like an alarm. I have an iPhone 4 and iPhone 5 doing that. However, it's not easy to find an app for such a use, I'm planning to open-source my custom one + setup.

However, BE CAREFUL, devices with batteries MAY BE DANGEROUS. I have had so many bulging/swollen batteries, even with devices just stored in a drawer. No incident or fires yet fortunately. So always keep an eye on them!

I run repair shops for a living, so I can tell you the #1 thing people do is give them to their kids or other family members.

Trade-ins with carriers are big as well, and of course selling them will still net some money as well. You can sell to local stores or online at sites like Swappa.

People who aren't as technical as the typical HN reader often don't know that iPhone, iPad, and laptop batteries are replaceable, and it's usually relatively cheap to do so. Please educate your friends and family members!

I always look for that one friend who still has a really bad/old smartphone and give it to them for free.

Old iPhones make excellent portable music players. Use them yourself in the patio, car, bathroom, etc.

Donate to others who would have a need.

I donated my favorite iPhone5s (the best iPhone ever) to my dedicated home helper (yeah ... they exist in this part of the world!)

Donate. There are a lot of organizations that collect secondhand phones for people in need, including the homeless, refugees, people escaping domestic abuse, and the like.

Wipe thoroughly first, of course.

Depending on how old, sell them on eBay. They have okay resell value, you get money and keep a phone out of a landfill.

I use them. I've actually never bought a smart phone (nor gotten one for "free" due to some contract) because there seem to be so many people willing to stop using functional phones. Currently I'm using an iPhone 5c and don't have any upgrade plans anytime soon.

I gave them to my parents. I also still have an old iPhone 5 that I bring to the rock climbing gym and use on the WiFi because I won’t be as upset if it plummets to its death from my (zipper) pocket or gets crushed somehow.

With the ability to receive phone calls on linked devices over WiFi, the old phones are still useful where the risk to the device is unusually high.

I have an old iPad2 and I'm honestly amazed at how bricked it is at this point just from the passage of time.

Recently I turned it on after a long while, asking myself this post's exact question.

I went into the browser, but I got SSL errors accessing Google, Apple, etc. Okay...

Then I went to the settings app to see if there are updates, but the device is obviously unsupported at this point.

Finally I went to the App Store to try and install some picture frame app or something, but that was stupid of me since I already knew Apple certs were no longer valid in my obsolete OS.

Bottom line: It's trash with a touch screen. Galaxies are much better in that aspect. Rooting them provides a world of possibilities.

We've had an iPhone 3gs (30-pin) plugged into a speaker playing music in our son's room for years. He plays the violin, so listening to the piece he's working on and previous pieces is useful and calming for him.

Old iPhones:

1) Make excellent phones.

2) Make great music players.

3) Make great backup phones to use when hiking, biking, running, etc.

4) Make great GPS devices for older cars (Some maps apps let you download maps to be used offline).

I would like to use mine as a "pay as you go backup". But I don't know a service that would let me do that without any monthly fee.

I connect my old phone to my speakers during parties as a kind of streaming device. It's easy to control them from my main phone

I had to use my old iPhone 6 as a backup recently when water got into my 6S and the touch screen became erratic and had dead spots on it. Took a few days for it to get repaired (sister-in-law's husband did it for free so I wasn't going to complain about how long it took) so I had to use the old one during that time.

I mostly only get new ones when there are serious issues with the old one, so generally they just get recycled. My first, a 3GS, did last many years with a white noise app as a overqualified noise machine in the kids' room but even it is finally dead. One of my wife's was still in good condition at one point and we did resell it.

I use my old iPhone as an international travel phone. I keep the Messages, Mail apps, but add things like WhatsApp, VPN, Skype, Google Translator, Measurement Converters, GoEuro for use when I travel overseas. Bonus points if you have to turn it on when you get back to the US and it doens't have many apps on it.

I pass them down to my kids as iPod Touches. Then I mostly forget about the previous generation.

I'm hesitant to replace their iPhone 6's with our iPhone 7's. It doesn't feel like an upgrade. Our 7's have had issues since the beginning, and the headphone jack is very valuable to the kids.

Give it to someone from the Third World, chances are, it'll change their love more than you would imagine.

They mostly collect dust as backup phones. But it feels better as insurance than selling them away for a couple hundred bucks a pop.

Maybe I'll put the Hue app on one so I have spare remote controls for lights.

It would be nice if I could use an old iPhone as a hub for HomeKit, but I think it only supports iPads.

I've jailbroken a couple and given a few more away to family that otherwise wouldn't have paid to upgrade.

Would love to run little servers and other dumb stuff on them, but I never have. Always felt like it'd be fun to run a server/Python process on my older phones.

I made a Myst linking book like this https://learn.sparkfun.com/tutorials/myst-linking-book but it was way easier to do with a smartphone.

I thought about using mine as a Fire TV remote because the one that comes with it is so bad, but the battery life just isn't good enough (and I don't want to have to always plug it in after use - I keep my remotes in a tray inside my coffee table).

I have several phones doing sterling duty as paperweights and look forward to suggestions. One disappointment of our market system has been the failure to produce new apps designed to run on older hardware.

I created an itunes account for my daughter when she was two months old. The phone stays on a charger in her room and is used for nothing but playing apple music when she's sleeping.