What ways can a person contribute to significant science without being a full-time professional scientist?
I'm interested in hearing any possibilities that could fit a broad interpretation of the question, and compiling a list that could be useful to a wide range of people.
To stimulate your thinking...
"Person" could mean an interested layperson with a few hours to spare at one end of the spectrum, or at the other end it could mean a former research scientist who now works in some other profession, and could put in significant effort given the right opportunity.
"Science" can be taken as broadly as you want, including physical sciences, life sciences and social sciences.
"Contribution" could mean anything from spending an hour on Galaxy Zoo
on up. It could even mean taking up a career that contributes directly to some field of science, but using existing skills that came from some other field. (E.g. I got the sense from some other Mefi questions
that it might be possible to work in computational biology based on programming and data analysis skills without having deep training in biology.)
If it's something that would have a person feel they helped advance science in some way, and it doesn't require going back to college for several years to do it, it probably counts.
Please also explain what skills and resources people would need, where that's not obvious.