Looking back one, two and five years ago today on Worldchanging:

2009
Ed Mazria's Two-Year, Nine-Million-Jobs Investment Plan
Ed Mazria, founder of the non-profit research and education organization Architecture 2030, put forward a proposal to combat two urgent economic crises with one plan that will not only relieve financially strapped homeowners and generate job opportunities, but also leave us with a more resilient, more efficient built environment when the US economy recovers...

2008
Green Buildings and Smart Grids
(OK, this was technically posted on the 14th, but since there was nothing on the 15th, and this is worth revisiting, we hope you'll forgive the slight license)

Patrick Mazza reports on emerging energy ecosystems that connect smart, green buildings with a smart, green grid to optimize energy flows. Since commercial and industrial buildings represent around 40 percent of U.S. energy use, and homes another 30 percent, this represents the most significant opportunity for energy efficiency and mass-scale renewable generation...

2005
Biodiversity Farming and Restoration for Profit
Blogs, SNS's, tagging and other social tools are discussed back before social media giants like Facebook and Twitter were widespread...

Other recent "look backs":
April 12
April 13
April 14