NY Cares is a fantastic organization. Its model has been so successful that it has been emulated in over 60 other places in the US. In addition to the noble reasons for volunteering, NY Cares makes it very easy. There is a HUGE variety of projects to choose from, and you have flexibility--time of day, day of week, type of project, etc. Plus, all of the projects are in places close to public transportation. \r
Break out of your shell, meet some good people, see parts of the city you're not accustomed to or stick to what you know. Just get involved!
Pros: flexibility & choice
Cons: projects fill up fast