There is a great post in the ALSC Blog about parenting programs at the library. If you can find the right topic and the right night, you will end up with a surprising number of parents who are hungry for information and resources!
I used to coordinate events at a family resource center, and I was astonished at the number of parents who would sign up--and then show up-- for a Parenting Your Spirited Child session I ran with the county extension agent, or a series of discussions based on the book How to Talk So Your Kids Will Listen and Listen So Your Kids Will Talk, or one fabulous, huge parenting extravaganza with multiple speakers that a bunch of agencies collaborated on. Providing free childcare really helped, but parents were so ready for a chance to learn and discuss parenting issues. Is there someone in your community already doing this, or someone you could collaborate with to do it at the library? I bet there is.