10:10 campaign.

I happened across the launch day in 2009 whilst visiting the Tate Modern in London. It was held in the fabulous turbine hall - plenty of room for stalls, speakers etc.

From Wiki:

10:10 is a global warming mitigation campaign calling for immediate reductions in carbon public support, apply pressure to policymakers to commit to national cuts, and inspire success at the United Nations climate change negotiations.

As of June 2011, over 100,000 individuals, businesses, schools and organisations have joined the campaign and pledged to reduce their emissions by 10% in a year.