VolunteerSpot and SignUpGenius are two sites that help you organize help, resources and donations for an event. Both let you invite and track volunteers for an event, plus keep track of tasks, contributions and other event details. VolunteerSpot also has a cool tablet app called Clipboard so you can manage signups and volunteer activities on-site.
Both sites are free for robust capabilities that will take care of the needs of most organizers, but you can upgrade to manage more complex events or to have an ad-free environment.
