I use Wikipedia as my source for quick facts, general outlines, and links to helpful materials.
I mostly read articles related to the news feed or to my area of study, which is cell and molecular biology. I edit mainly spelling and mechanics stuff. Occasionally I contribute information, correct factual errors, or add links.
I'm mostly a fly on the wall here because I don't have oodles of extra time to spend researching and writing articles, though this is kind of what I'd like to do.