During the last few years, there have been a few occasions when I've been at a computer but without immediate access to a periodic table. My standby is Web Elements, a site that seems to have been around forever.
I recently happened onto Jorge Goncalves' Triple Point, which summarizes the best 55 online periodic tables. Although some of the links are broken, there's plenty to choose from if Web Elements doesn't do the job.
The various forms of elemental periodic tables are in many ways the ultimate chemical information systems. They summarize what's already known and even after a century still light the way to new areas of discovery. Periodic tables demonstrate, like perhaps nothing else in chemistry, the power of organizing information.
Image Credit: Mike Mertz