From Wikipedia:

Citizen science (also known as crowd science, crowd-sourced science, or networked science) is scientific research conducted, in whole or in part, by amateur or nonprofessional scientists, often by crowdsourcing. Formally, citizen science has been defined as "the systematic collection and analysis of data; development of technology; testing of natural phenomena; and the dissemination of these activities by researchers on a primarily avocational basis"…

Over the years I've been involved in a few Citizen Science initatives:

When I was in university, I signed up for the Ontario Tree Atlas Project. Alas, the quadrant I was assigned ended up being part of a HUGE wetland in my area. I contacted the local Conservation Authority to find out if I could tag along with their staff to do my surveying, and they said they were sending a crew in that winter. However, it was an uncommonly warm winter, and the wetland never froze over, so I never did actually collect any data.

Back when I worked for Ontario Parks, I got involved in a couple of different Monarch Butterfly programs. I set up a plot and got park visitors to help me collect data on a weekly basis for the Monarch Larva Monitoring Project. In the late summer, I also got park visitors involved in tagging Monarch butterflies for the migration, through Monarch Watch. Three of the Monarchs that were tagged during those programs were later recovered and reported to Monarch Watch, which is kinda cool.

I am also a volunteer with the Dr. Susan Love Foundation's Army of Women. After joining the Army of Women, volunteers receive regular email updates about Breast Cancer researchers looking for subjects to take part in Breast Cancer Research studies, and can sign up to join those studies if they meet the criteria. This doesn't strictly meet the criteria of Citizen Science as laid out in the Wiki article I cited, because people are volunteering to be subjects, rather than doing the science themselves, but I think it's still in the spirit of the idea.

And then there's SETI@Home. Mr. Ivriniel has his desktop running this program, which has people all over the world donating unused computer processing time to analyzing radio telescope data.

What about everyone else? Anyone have a Citizen Science project they take part in?