For organising weekly church rosters, I still highly recommend Planning Center Online. However, churches have lots of other irregular events that require volunteers to either bring things or do things. VolunteerSpot is a service that makes it easier for people to sign-up (with a really easy interface for smart phones, too). According to VolunteerSpot:

"VolunteerSpot makes signing up to help so simple, most groups see a 20 percent increase in new volunteers at their VolunteerSpot-powered events."

Here's a brief overview:

This is a more detailed explanation of how it works:

In terms of pricing, there's 2 different options - free and premium:

Find out more at VolunteerSpot. Is this something your church would use? Why or why not? What would make this service even more helpful?