Do you know ReactOS? It's an open-source, free operating system, running Windows applications. It's still in alpha stage, but runs a lot of programs well.
One of the developers asked us to make their embroidered t-shirts, and as usual, we couldn't say no. ReactOS is not Linux-based, however, but free - as in freedom.
A very popular free software's official shirts are in the pipeline too. Stay tuned!