You are allowed to put ANY volunteering in your resume, online or onsite. Since you haven't had an ongoing role at any organizations as a volunteer, then list the work as Pro Bono Consulting/Volunteering, and then the types of projects you undertook, for whom, and the dates. For instance:
Pro Bono Consulting/Volunteering
- Translating a brochure from English to Spanish for Help Everyone Inc., a nonprofit in Phoenix, AZ. Jan. 2010
- Research various topics online and gathering information for Children Matter, a government program at the Joe Smith Memorial Park in St. Louis, MO. Dec. 2009
- Editing newsletter articles from other volunteers for Spirits Museum, a nonprofit in Chicago, IL. Nov. 2009
You don't have to say it was online volunteering - or you can, if you think that might be particularly interesting to employers (I know of two people who have been asked about their virtual volunteering on their CVs during interviews - both got the jobs!).