This is a work in progress. Please feel free to suggest sites and stay-tuned as more sites are added.
This is a great place for families to go and look up cool earth science sites in their area or in areas they may visit for a vacation. These sites function like geocaches, except that you can learn about the natural processes that formed/influence the site you're visiting. Visit a bunch and you can earn patches through the Earth Cache Master Program. It's a ton of fun.
Monarch Watch tags thousands of butterflies every year as part of a program to better understand the Monarch's habitats and migration patterns. There are a variety of ways you can get involved from growing Monarchs, tagging them, or even creating a butterfly habitat so they can stop off for some food on the long journey to and from Mexico.
This is a great resource for parents and teachers who are looking for hand-on inquiry based science activities. They provide background on how to do the activity and why it matters. You can upload your data upon completion. It covers a wide variety of ages.
The National After School Science Directory
This site allows you to search by zip code and find science activities in your area. This is great if you want to get involved and don't know where to start, you've moved to a new area, or you're just looking for something different. It covers a variety of age ranges and science, technology, engineering and math (STEM) activities. Thanks to @ComplexLifeNAI for letting me know about this.