An actual example of a free lunch.
I put solar panels on my roof, they cost me so the energy isnt free, but I bought them to power my home. They often produce more power than I can use.
The excess power runs my computer, my computer in its spare capacity runs BOINC projects. The BOINC projects are using my computer to solve humanitarian problems like disease research, which are too expensive to run on paid resources.
My computer runs cryptocurrencies like Gridcoin and Solarcoin, that I often give away small amounts of.
Free lunches come from re-purposing resources, that on their own have no other valuable use, and therefore have no opportunity cost.
Join me, give.