Kalzium provides you with all kinds of information about the Periodic Table of the Elements. You can lookup lots of information about the elements and also use visualizations to show them. It is free and licensed under the GNU Public License.
You can visualize the Periodic Table of the Elements by groups, blocks, and families. You can plot data for a range of elements for properties like the boiling point or the atomic mass. You can go back in time and see what elements were known at a given date. You can also calculate the molecular mass of molecules.