TIJEJ  This software is stucked in the Mac OS 9 ages.

If you want a modern periodic table (not free, but quite cheap : $10), check out Elements from Digital Science.  
(Version 4.0.8)

UMIJIN  Try Smell-O-Mints for a *free* periodic table with many of the same features.

I'm not sure Periodic Table X gives you that much more for $20, but it certainly looks nice.  
(Version 4.0.8)

ANONYMOUS  There is a free widget and it is as good as this shareware.  
(Version 4.0.1)

MEL BECKMAN  As an amateur biochemist, I've scoured the Internet looking for the best chemistry tools. This is by far the best periodic table tool for the Mac. Its startup preferences let you configure the program to display your favorite element attributes at launch, and the thing is chock full of properties and other helpful information. There are some other programs out there that are a little prettier, but they all seem to sacrifice utility for cuteness. PeriodicTable is a workhorse!  
(Version 3.5.2)

MARK ROSS, PH.D.  As a professional chemist with 25 years working in the field, this is the one I use. Its ability to make tables of properities for any of the parameters it has is priceless. You can quickly see the relationships between all the elements that greatly simplfy determining which will give the desired result. Great job!  
(Version 3.5)

ANONYMOUS  Really good app, very useful for High School Chemisty.  
(Version 3.4)