I am fortunate enough to live 5 minutes from our city's zoo. While the cost is reasonable for a day trip, ($8 per adult, $5 per child) that makes a visit $21 for our family.
My parents bought my daughter a zoo pass for her birthday. It was $49. It's good for a year and we can go as many times as we like. Actually the pass allows me as well as another adult (It doesn't need to even be my husband) and as many children as we bring, so this is a nice thing I can do with friends with kids as well.
The nice thing is that being so close to the zoo we can go and pop in for an hour and then be home for lunch without having to spend an entire morning there to get our money's worth. We also don't buy any of the unhealthy overpriced lunch items they sell in the cafe as we never eat there. (although there are some nice picnic places there as well.)
So if there is a place you visit often (or would like to) looking into a family pass is a great way to save money. It was nice to go this morning and not have to spend a dime!