Volunteer Spot is a free scheduling service that teachers, coaches, and others and use to coordinate volunteers. Volunteer Spot gives you the ability to post calendars and sign-up sheets online. When potential volunteers visitor your calendar they can see the days and times at which volunteers are needed. When the quota for volunteers for a particular time or day is reached, Volunteer Spot won’t allow any more sign-ups.

Applications for Education
Volunteer Spot could be a significant improvement over trying to keep track of emails from parents. You could also coordinate sign-ups with a wiki or similar tool, but getting parents to use Volunteer Spot is probably much easier than trying to convince a whole class worth of parents to join a wiki.

If Volunteer Spot is more than you need in a sign-up application, you may want to look at SignApp Now.

Update: Just received this great news via a comment from Michael Walter.
After using it for only 3 days, parents in my school are raving about it. It made scheduling a diverse group of volunteers incredibly easy, mostly because they scheduled themselves. Parents were in love with the easy interface, and the email reminders, the only thing I was worried about, were a BIG hit. Going to try and get this expanded beyond just my library helpers, and bring it to my next collegiate circle to share.
Michael, thanks for sharing your experience with us!

I originally learned about Volunteer Spot on iLearn Technology which is a fantastic blog to visit if you’re looking for resources for elementary school settings.