[yyyk]

>Sure, but if the machine is already running, it's free.

It still has costs for admin and electricity (via extra CPU load), though I grant these could be marginal.

>Or if you can stick it in a cupboard at work/university, it's free.

This is using someone else's resources, rather dubiously ethical. I suspect IT is going to have a fit when they find out - assuming they didn't block the relevant ports in the first place.

>is there actually free cloud hosting? don't they ask for your credit card first before they offer you "credits"?

There are free plans on Azure/AWS/GCP. Asking for credit card data does not mean you are being charged, I suspect it's really more about identifying users.

It's possible to set up free plans without credit card if one has some other id (e.g. Azure's free plans via MSDN). A prepaid card should work as well.