Ask your boss to let employees work from home

This may not apply if you're self-employed, work freelance, or have a non-desk job, but for  plenty of people, transport, energy and other costs could be easily reduced by allowing employees to work from home one or two days a week. And this doesn't even include considerations of worker well-being and productivity (take note, employers). Really, constant human contact in the workplace is overrated in this era of Skype.

Effort involved: all you have to do is raise the issue. You can prepare a detailed analysis of the environmental and economic savings if you're feeling industrious, or if you're at your desk and bored.