I thought I heard Brazil had open source tax software. It would need to be sponsored and managed by the government in order to work. With open source software the liability is on the end user and a corrupted Word document is a bearable risk. Businesses pay for Red Had and system engineers to reduce the risk their systems won't boot. In addition, tax laws change all the time (and are honestly a bit subjective).

I would imagine it would have to be managed by the government (or someone with liability) but accept contributions that would be vetted and approved by the government.