In the past I have written about using Ubuntu and other Linux distributions to give old PCs new life and to replace Windows when a valid licence or other issue prevents installation.
But I haven't mentioned using Ubuntu on Macs before.
Recently I was given an iMac to securely erase and donate to charity.
This machine was too old and had too many problems to be able to install any valid Apple operating system.
It would note take anywhere near the latest version of MacOS and it wouldn't even put back on the one it had been running.
Apple are particularly ruthless about end-of-life for operating systems on older machines.
It wasn't worth wasting my time on. Even if I had won the battle the result would be an old machine with a bloated, outdated and insecure operating system.
So I just whacked in my standard Ubuntu installation thumb drive and gave it a go.
Booting from a thumb drive on a Mac is different but easy.
If you've used Ubuntu before then everything will be incredibly familiar. I mean, it's running from the same thumb drive you use to install to your PC...of course it is going to be the same.
Much like for old PCs, Ubuntu asks a few questions along the way and then simply gets on with everything.
At the end of it you are left with a fully functioning machine on a current operating system.
There's a chance you'll bump up against more problems than you would using a PC, but I didn't.
This iMac came up fine and everything worked.
You may want to try some lighter versions of Linux on (much) older Macs. I found Ubuntu slightly sluggish but this was predominantly due to the crappy hard disks Apple tended to put in a lot of their iMacs (read, terribly slow performance - not really Ubuntu's fault).
NOTE: If you can be bothered, because it isn't a trouble free upgrade on for many iMacs and Macbooks, slipping a Solid State Drive in place of the original hard disk will drastically improve performance. Check out iFixit for details before assuming you can and want to do that.
Linux Mint appears to be the favourite light distribution for Macs but here's a list for you anyway: https://www.slant.co/topics/15398/~linux-distributions-for-mac
I've used Mint before and it is fine. I can't vouch for any of the others listed.
Depending on your needs this can be a great way to keep an old computer out of landfill and money in your pocket.
If your needs are simple I reckon you may just like this.