I ran an XMPP chat server from an old laptop for about a year (switched to a VPS for reliability though).

My friend buys and restores old ThinkPads and sends them to the Ukrainian military (Army SOS program)

But I still have several laptops and smartphones laying around. A long time ago, I had an idea of running a pseudo-datacenter with low-spec devices like that, but I think that would be more of a responsibility and money sink. My coworker suggested using those devices for additional calculation power (for lambdas?) on cloud services during spikes. But I haven’t really looked into that.

For makerspaces, old laptops and smartphones can run octoprint for remotely controlling 3D printers.