I'm not exactly sure if the tool I picked to discuss is really considered a web tool but it's really the only technology I use that is not required curriculum through my district/school site.
Mystery Science is a website that you need a subscription to access. Because we don't have a "true" science curriculum this works and an awesome supplement . You can search the site for specific lessons and experiments on whatever unit your class is learning. My favorite component of Mystery Science is Mystery Doug. Doug is a teacher that lives in San Francisco who answers student's questions about science. Every week he has new science videos that are kid friendly and are on topics that can also be found in the standards or very closely related. At the end of each video he offers fun, strange and unusual facts about the same topic. My students love it!
The only con would be the materials you need for the science experiments are not provided so you have to go out and find them yourself and that can get expensive and/or time consuming.