I just had a crazy, most likely stupid idea, but I figured Id just throw it out there. A lot of people spend time contributing to forums answering peoples questions when if a client asked them the same question, they would get billed for it. Could you mark off contributing to forums as probono consulting gig? haha However, honestly, I don't see why you couldnt do this if you kept accurate records of your time spent answering peoples questions in a field that you work in. I wonder if people who work on open source projects such as Drupal, Joomla, etc. mark off the time they put into the project as tax deductible.
I don't think so because the Tax Department has no way of checking up to make sure you DID actually contribute to a forum and not just make it up.
At least in the US, something is only tax deductible to the extent that the IRS defines it as such. Volunteering or even handing out $100 bills on the street corner may be a nice thing to do, but it's not tax deductible. Only contributions (in cash or time/product/services) to legally qualified non-profit organizations that are registered as 501(c)3 corporations would enable you to write them off on your state and federal income taxes.