Science can produce some very cool things on the Web. One of them being this ridiculously useful Web version of the periodic table of the elements--a staple for chemists and scientists at large. The chart, found at Ptable.com, is completely dynamic, letting you adjust nearly every aspect of the data to see what each element does at various temperatures, and even turn back the hands of time to see which parts of the chart were missing before being discovered by scientists.
The table is hooked up directly to Wikipedia, letting you view the encyclopedia entries as small pop-ups that are skinned to match the site. You can also drill down on any element's info and view the data with color coding based on which group the element resides.
I'll be honest, the last time I saw this thing I was taking a prerequisite chem class back in college, but it's far more utilitarian than anything you'd find in a text book, which makes it a great tool for students. There are some other Web-based tables out there, but none of them I've seen look and function nearly as well. It's also one of the oldest, having launched back in 1997.