Dynamic/Interactive Periodic Table (ptable.com)

I use this all the time in my chemistry class when I don't feel like exerting the effort to open my book. It's superbly helpful.

To really enjoy it, click the Properties tab, then choose Boiling point or Discovered, and _then_ operate the slider. Provides a good light show in modern browsers. Clicking on elements in the Isotopes tab is good, too. Also the Wide checkbox at top left. Hell, just click Demo at the bottom and take in the show. No Flash, either.