I've asked this myself sometimes already. A lot of knowledgeable developers are using Stackoverflow to give advice and help solving other peoples problems - for free. Giving back your own knowledge and helping students, newcomers or just less experienced developers can be a really good thing. It's a great feeling to know that you've helped someone.
I wonder if people would also be up for voluntary (donations at maximum) mentoring other people - face to face aswell (assuming the person mentored is actually intending to learn and not just trying get help on a project for free from an expert).
I know there are sites like codementor, but they are all based on a fixed hourly rate - you can basically only hire a mentor/expert.