Looking for volunteer management software…?

I’m trying to find a piece of web-based software that helps with managing volunteers. It would need to be something that has a login system so that volunteers could be scheduled for a specific event by a coordinator, and then the volunteers could login to approve if they available to help. Any ideas?

What Volunteer Management Software/System are you using?

Are there any quality volunteer management software systems that allow for easy tracking, scheduling, confirming, with the additional ability to print statistics of number of volunteers, hours, etc.?
Preferably free programs?
Has anyone heard of “Service Monkey’?