Science NetLinks is a great resource for science teachers (and any other teacher looking for science information and resources). They have resources for grades K-12. Resources include lesson plans, online resources, lesson tools, and more. It is all free!
The resources are all standards based, internet activities. I have been using a few of them and they are very good.
The lesson plans section is set up by grade level. You can scan the list for the topic and standard benchmark. The tools are links to different web sites that you can use in your own lesson plans. The resources section has a list of resources, sorted by topic and grade level, such as NASA, American Museum of Natural History, and much, much more.
I found the site to be easy to navigate and the lessons and resources were all good quality.