I'm surprised nobody's mentioned Theodore Gray's periodic table table. Possibly not the best research tool, but an excellent general interest site (and a great one for getting kids interested. He basically built a periodic table table out of wood and stores much of his element collection in it.www.periodictabletable.com
It's also interactive but has photos, stories, videos and rotating images of his own samples. He won the 2002 Ig Nobel prize for chemistry for the table itself.
Theo writes for Popular Science magazine and has also published his own paper periodic tables (which are the best I've seen) a few books and a deck of periodic table cards (one for each element).
WebElements ( www.webelements.com
)is one that has been around as long as I can remember and has a lot of information about each element. As well as interesting information about discovery, uses etc, it aqlso has a lot of physical and chemical data and is a useful reference tool.
...and for sheer beauty, the RSC's "Visual element" table has a piece of digital art for each element created by Murray Robertson. http://www.rsc.org/periodic-table
(then click on Visual Elements Images on the top left). The normal table is a good interactive one as well, and has a slider so you can see the states of elements at any temperature.