www.clubfreetime.com is a comprehensive website of free activities in New York City. It will list everything from free philharmonic concert listings to free student recitals to free chamber concert series of small companies. There is a minimal fee- $1.95 for a week, $2.95 for a month, $20 for a year membership. With the membership too it offers VERY discounted tickets to events. It is REALLY worth it because again, it lists a great deal of free or low cost activities. Other free events are listed too (like museum tours), but you can narrow your search to music and it lists events daily. I saw a GREAT free chamber orchestra event recently at the french institute with my boyfriend and there is NO way we would have known about it otherwise because it was such an obscure date and time for this event.
Hope that helps!