Actually, you can save and enhance many lives. To become an organ donor after you pass, you can simply register at http://www.donatelife.net. Please make sure you let your family know your wishes as this makes it much easier for them emotionally.
Living donation is also an option for some organs. For more information, see http://www.unos.org/donation/living-donation/
An easy way to help others is by donating blood. The Red Cross is an excellent resource and organization.