Many of us do weekend 5K races that benefit charities. Some of us train with charities for longer races or triathlons. I found a site that lets you log your fitness activity every day and convert that time and effort into a charitable contribution. There is no cost to you at all. There are corporate sponsors donating money to the charities for the activities that you log. Today, I raised $1.63 for the Go Red for Women campaign from the American Heart Society. No, it's not a huge amount of money, but it is $1.63 more than they had before I signed up.
For people who aren't internally motivated to be active and need to feel they're helping others, this is a way to get that external motivation. For those of us who thrive on competitive, there are leader boards and challenges on the site. You have nothing to lose, so check it out.
For those of you who want to train with a charity to run a half marathon, full marathon or a triathlon, join "Team Twitter" with Team to End AIDS.