The NSTA (National Science Teachers Association) has a great resource on their web page, called the Science Teacher's Grab Bag. If you filter by cost "free" it comes up with close to 100 free resources for science teachers. You can also filter by different dollar amounts if you have funds to spend on resources. You do not have to be a member to access this site.
In general, NSTA is a great resource for teachers and I always recommend that science teachers join. Your membership gets you access to more resources, discounts on NSTA publications, and includes a subscription to a journal of your choice.