We use signup.com to coordinate our volunteers.  It's not perfect, but they're getting better and better at their work.  The ads are not my favorite, but they aren't too obnoxious.  No pop-ups.  The interface is pretty simple, and there's a lot of flexibility.  You can even copy previous days for efficiency.  We particularly like the reporting options, which allow us to report our volunteers' hours to our local co-op for their member/worker discount (our co-op offers 7-12% off on groceries for people who volunteer for approved non-profits).  We pay for a plan, but I believe they have free plans, depending on how many volunteers you'll be coordinating.