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Imagine asking today how the Internet affects business. It's an absurd question, like asking how electricity changed business. Asking the same about sustainability, it turns out, is equally absurd. Like the Internet, sustainability spurs innovation in everything, from how you see your business model to whether you see your employees (why not let them work at home more?). Here are our favorite ways companies today are greening up--and saving money and making better widgets in the process.

1 At $100 a ton, feeding a landfill is pricey. But in the past two years, General Mills has turned its solid waste into profits. Take its oat hulls, a Cheerios by-product. The company used to pay to have them hauled off, but realized they could be burned as fuel. Now customers compete to buy the stuff. In 2006, General Mills recycled 86% of its solid waste, earning more from that than it spent on disposal.

2 Moore's Law is great for producing speedier devices, but it's hell on the environment. According to Greenpeace, demand for new technology creates 4,000 tons of e-waste an hour, which often ends up on dead-hardware mountains in India, Africa, and China. Enter take-back programs, in which customers return spent technology to manufacturers, who recycle the parts for new gadgets. The United States has long lagged behind many European nations, which mandate the programs, but that's finally changing. Dell is leading the way. Last year, the PC maker recovered 40,000 tons of unwanted equipment for recycling, up 93% from 2005.

3 Trains were already the cleanest way to move massive amounts of freight long distances, but General Electric raised the game with its Evolution locomotives, diesel engines launched in 2005 that cut fuel consumption by 5% and emissions by 40% compared to locomotives built just a year earlier. Up next, a triumph of sheer coolness: a GE hybrid diesel-electric locomotive that, just like your Prius, captures energy from braking and will improve mileage another 10%. According to GE, the energy dissipated in braking a 207-ton locomotive during the course of a year is enough to power 160 homes for the same period.

4 Not to be outdone in the freight game, Wal-Mart is providing funding to the biggest truck manufacturers--ArvinMeritor, Eaton, International, and Peterbilt--to develop the first heavy-duty diesel-hybrid 18-wheeler. Wal-Mart, which operates the second-largest truck fleet in the country, will test the prototypes next year.