I've referenced this page for a while when planning out new projects and prioritizing them. BuildItSolar.com has an excellent list of conservation projects. He did this as part of his '
"Half Project" which the objective was to half his energy useage. The list is very well put together with listed costs, energy savings, money savings, CO2 reduction, ROIs (return on investement), and difficulty level. Some of the projects are surprisingly simple and cost next to nothing. Some are a bit more expensive. My personal favorite was the duct sealing. 375% ROI in the first year! Its hard to be that.