You can put anything on your resume that you want. There is no Resume Police. You should put on your resume anything that will make the reader say "Hey, I need to bring this person in for an interview." Conversely, you should not put anything on your resume that does NOT make the reader say that. Your two summers at McDonald's? Don't bother, even though it's paid work experience. Blog post on the topic: Should I put _____ on my resume?.

With any experience on a resume, you'll want to quantify it as much as possible. Compare: "Uncovered and documented bugs on Firefox" with "Uncovered and documented 47 bugs in Firefox over a six month period." The latter gives the reader a better idea what it is you've done. More on using numbers in your resume: Numbers and resumes.

Where did you get the idea that it could only be paid experience? Did something tell you that? If so, I'd love to find out what book or blog told you so that I can bookmark it as bad advice.