A lot of Americans go into service professions, but most have other means of employment, and need to find ways to help that complement their lives. To be really fair, this requires time and/or money that might be lacking; for example, a single parent putting kids through college is already more than busy. Here, I'm focusing on people who might have a little extra.
Here are four possible aspects of "a craigslist for service."
1. If you have the time and inclination to get out, you might volunteer for an existing service organization, probably a recognized for-profit. There are sites which make this relatively easy, the most effective of which is VolunteerMatch.org.
2. You might have some cash you'd like to pool with others to get something done. Sites which make that happen include DonorsChoose.org, funding classroom projects, or Kiva.org, which provides micro-finance loans to small businesspeople.
3. You might have the time for traditional civic engagement, where you participate in local governance. For example, you might join the PTA, or just attend local city council or board of education meetings, or join the board of a small non-profit. That's traditional grass-roots democracy, an important American tradition.