Van Leeuwenhoek's primary journals are found scanned within this 516 page eBook. For me, far aside from content... I would use this resource when coaching students on interactive science notebooks. His handwritten/drawn documentation is still a model today. To day we could use a digital camera, back then... he WAS a "camera." So cool.
The emerging set of interactive simulations at PhET that now use HTML5 and work solely within a modern browser. These are heavy in the physical sciences... but also include acid-base, atomic structure, chemical equations, etc.
i tried this demo for the first time this year. It didn't work perfectly but it really gave the students a visual of the formation of minerals. I would suggest marking the water level on the jar when you first set it up so they can really grasp how much has evaporated by the end. I let mine sit for almost 3 weeks but crystals were forming within 3 days.
I think a key element to the credibility of this set of resources is that they were curated/produced by the Science Education Resource Center (SERC) at Carleton College and its partners with funding from the National Science Foundation.