I'd also recommend Boston Cares. I'm actually volunteering at the Food Bank with them this afternoon. They have a really diverse offering of projects and are great for when you have some spare time and want to do something good with it.
Location: Places >> North America >> United States >> Massachusetts >> Boston >> Boston Volunteers
here's a volunteer organization that has some great one time opportunities for those who have really busy schedules.
They have a calendar of events each month, so you can quickly sign up for a volunteer event you would like to attend. There is a one hour orientation for new members too. It's a lot of fun- recommended :)