I always start by checking my state standards page for science and the NGSS website to help further understand the standards.

I like Mystery Science and Generation Genius for STEEL standard ideas.

I agree with all the great resources that have already been mentioned!

One resource I used when I taught science was The Science Penguin (https://thesciencepenguin.com/science-penguin-library-landing). I love her blog and you can sign up for her free resource library. I also follow her on Instagram.

Home Science Tools has a wide selection of experiments: https://learning-center.homesciencetools.com

I also love the Smithsonian site for all sorts of information: https://ssec.si.edu

The eagle nest cams got my 4th grade scientists so excited! https://eaglesorg.wpengine.com/what-we-do/educate/live-hd-nest-cams/

I use Sid the Science kid videos on youtube to help teach lessons. My students like the songs that they sing and the science experiments. I also use Generation Genius because of their Kahoot games that help check for learning. I use Mystery Science to perform science experiments after watching the informational videos.

My absolute favorite science website is The Wonder of Science, https://thewonderofscience.com/
It is aligned to the NGSS standards and is phenomena based learning.