If it's time to replace or upgrade your mower, you can find a number of greener options on the market. One energy-conscious option is the reel mower, which transfers the power of the person pushing to the set of spinning blades that cut the grass. Since physical effort is what makes them go, manually powered reel mowers are best for lawns that are both very small and very flat.
You don't have to give up on power to have a more environmentally friendly mower, though. A number of manufactures produce mowers that use alternative fuels. Electric mowers, for example, are quieter than gas mowers and don't give off exhaust. They're available in both cordless (rechargeable) and plug-in models. Cub Cadet also sells mowers that run on liquid propane, which also reduce emissions but don't need to be plugged in or recharged. You can also find mowers that run on diesel fuel and compressed natural gas.