"We don't donate to OSS software which we use, because we're legally not allowed to.
I routinely send key projects, particularly smaller projects, a request to quote me a commercial license of their project, with the explanation that I would accept a quote of $1,000 and that the commercial license can be their existing OSS license plus an invoice. My books suggest we've spent $3k on this in 2015. My bookkeeper, accountant, and the IRS/NTA are united on this issue: they don't care whether a software license is OSS or not. A $1k invoice is a $1k invoice; as a software company, I have virtually carte blanche to expense any software I think is reasonably required, and I think our OSS is reasonably required.
I would do this more often if OSS projects made it easier for me to do so. Getting me to pay $1,000 for software is easy; committing me to doing lots of admin work over the course of a week is less easy. Take a look at what e.g. http://sidekiq.org/ , which is an OSS project with a commercial model, does. Two clicks gets me to a credit card form. If I actually used Sidekiq, Mike would have had my credit card on file the day that form went up."