Tax incentives for private companies to fund academic research
The US has substantial research tax credits; in 2005, tax credit claims came to $6.6 billion USD, though it's not immediately stated what overall private R&amp;D expenditure was in that period. (I suspect it would not be too difficult to find out...). Similar programs exist in many countries; see this 2014 survey.

The Deloitte report helpfully summarises (pp. 54-55) whether subcontracted research is eligible under the various national laws (which would cover academic funding); for the US, they are but you get a reduced deduction, while for Austria (for example) there is a 1m Euro cap on the amount that can be claimed for subcontracted research, and in the Czech Republic it has to be in-house to qualify.