I have long been a fan of the Kraft Great Kids Museum Passes available through the Chicago Public Library (although these passes have become so popular at some branches that their availability is more theoretical than actual). These passes allow free admission for up to four people to some of the area's most popular destinations (like the Chicago Children's Museum, Museum of Contemporary Art, etc.)
Now that same type of pass is available in many suburbs. More than 100 suburban libraries offer Museum Adventure Passes to local destinations including The Brookfield Zoo, Kohl's Children's Musem and the Chicago Botanic Garden. Same rules apply -- free admission for up to four people and the passes must be returned within one week.
These passes can make an otherwise unaffordable family outing possible. Every family should be taking advantage of this great freebie. Check out this website to find out if your local library is participating.