It can vary slightly, depending on both the projects and their duration.
If you received any compensation for them, I'd list them under a single heading with the rest of my work experience, with a start date of the first project you freelanced for, and the end date of "present".
Leigh, Freelancing - July 1996 to present
Web development, copy writing, underwater basket weaving, monkey training, lion taming. Clients included sole proprietorships and microbusinesses in Some Metro Area, USA.
If they were volunteer opportunities, I'd put them under "Volunteer Projects" and list each project out with organization name, approximate start and stop dates, and a brief summary of what role you held/what you did.
Homeowner's Association Board - 2010 to 2012
- Treasurer, 2010
- President, 2011
- Seargant-at-Arms, 2012
How to list and how thoroughly to describe can also change slightly depending on what industry you're targeting, how long you've been in the workforce, and the relevance of the projects to the position(s) you're applying for. You can use your cover letter (assuming your local job market regularly makes use of cover letters) to briefly mention any relevant volunteer/side projects, so that when your resume is initially being scanned, they know to look specifically for that side project and take it into consideration when deciding whether or not to interview you.