Linux

Turn them into a little server that you can host self hostable services on

IMHO fix whatever you can, donate it all locally (HackerSpace, RepairCafe, Linux non-profit, etc) as there are quite a few people dedicated to refurbishing computers for schools, people who need a computer to find work, etc.

Then for the tinkering aspect, keep one, that's enough.

Honestly even 1 isn't really required. Pretty much everything listed here can be done more efficiently without an actual physical computer :

• your current computer can be a server, just turn off the screen or even accept (which I'd argue is a fair assumption) that at night it will be off. If you want external access put WireGuard or another VPN on it.
• Want to test distributions or anything else? QEMU or containers, no need for actual hardware

How old are these machines, from oldest CPU model to newest CPU model?

Put Linux on them and give them away to people who need them?

I have some hardware from like 2008 running my entire home's infrastructure. Jellyfin, Kavita, home automation, etc.

Repair what's broken, slap Linux on them and donate to charities.

Solar panels and cryptocurency mining

Cheap Linux desktops for a charity?

theres so many things to do with a pc that i dont know where to start

Pick the first project that you think of and chase it down. If it sucks, then reformat the drive and do something else. Video game systems and file servers are great. So is installing a different OS on each, just to experience the differences side by side.

Do NOT continue "analysis paralysis".

First of all: get rid of the broken ones. You’re not doing anything with the running systems, so there’s no need to hang on to the ones that don’t run.

Next, make a list of the things you want to do and start doing them.

If you’re worried about power consumption, don’t be. If you’re still worried about power consumption, get an inline watt meter (a kill-a-watt), take some measurements, do the math and feel at ease. If you don’t feel at ease, look up wake on lan. You can have powered down computers turn back on when they get a packet so you don’t need to worry about power consumption.

When you feel like you’ve done enough stuff, get rid of the computers you’re not using.

For any machines that are too inefficient to be worth continuing to compute with, you could at least save the power supplies for electronics projects. I've got some 12V addressable RGB Christmas lights being powered by an old ATX power supply, for example.

Some ideas:

• webserver (e.g. for a little personal website, maybe even host some fediverse things)
• irc
• weather monitor
• distro tester
• local LLM ~(they're getting more and more efficient)~

If you've several of similar performance, you could:

• host lan parties, for classic games. Maybe some Quake, OpenTTD, Luanti

Pretty sure there should be some nonprofit that will gladly get and assemble them so i.e. children on remote places can have a computer.

Explore weird OSs! I got an old Celeron D workstation just for playing around with weird old operating systems.

Its got a 32 bit bios but 64 bit celeron, so the grub stuff has been fund 😅

How old are we talking?

• Anything before core iX series is not recommended to be used as a server (missing instruction sets, low efficiency etc.).It could still be used for fun projects like installing gentoo or old redhat with plasma 2.
• If you have Core iX cpu (preferably 3rd gen or newer) you xan host some services, but look into c/selfhosted if you're interested in that.
• You could also experiment with Kubernetes and combine lots of bad PCs into one less bad PC.

In the end PCs are useful only if you can run useful sodtware on them, but besides nostalgia there ain't much use I see in them.

I recently turned every old junker and some nicer ones into a Harvester cluster. The really old ones I use as cold storage devices that I actually shut off when I don't need them.

A lot, depending on your interests and the hardware itself. I'm running a NAS (TrueNAS) on an old machine that also runs a bittorrent client and immich as TrueNAS "apps." I'm running an *arr stack and jellyfin on another old machine. I've got another old machine running an i2p router, hyphanet node, and a few other services. In the past, I've used old machines as routers (pfsense), openhab/home assistant machines, game servers, ZoneMinder server, etc.

A nas or home server with one of them is a great idea

Turn them into a Kebernetes or a Proxmox cluster.

If less than 10 years old they're probably usable with some upgrades here and there. Finding a use for them is harder. Maybe just get them working and give them to friends who can use them.

I'm in a similar boat. I use old computers for spare parts and hobby projects (e.g. I did Linux From Scratch on an old second-hand Thinkpad I picked up on a whim). I think cheap second hand computers are great for tinkerers e.g. you can flash custom firmware without worrying about bricking the mobo.

You could also use them as servers if you have any services you want to host.

Also if you truly have no use for them, fix them up, install something like Linux Mint on them, and give them away.

Install Win7 to make retro gaming machine?

You could make a Kubernetes cluster. Otherwise I don't think running multiple old computer really makes sense.

Make an awesome homeverver out of them! Cancel your streaming subscriptions

https://medium.com/linux-shots/self-host-media-stack-jellyfin-radarr-sonarr-jackett-transmission-3e6a0adf716e

If you want something out from the ordinary, maybe build a retro gaming/tech museum and make it open for the public. You can even host events like gaming competitions, or thematic presentations. Charge a small make up for the costs, and maybe you can even make it your living later in life.

What kind of machines are we talking about here?

A recent-ish tiny/mini/micro is a vastly different answer than a kaypro luggable.

Goodwill has a recycling partner that can take most e-waste .