GPeriodic is a program for looking up data of elements from the periodic table. It contains a broad data set for each atom that covers weight, density, melting pint, radius, appearance, and many more information. A command-line version is also included.
The Gnome Chemistry Utils provide a few Gtk programs and a C++ algorithm and widget library related to chemistry. It includes GChemPaint, a 2D chemical editor, and a chemical calculator for raw formule, molar weight, mass composition and isotopic patterns. It also comes with viewers for 3D molecule models, crystal structues, a spectrum viewer, and a periodic table of elements.
Lab Assistant is a chemistry experiment and tutoring application for Windows. It provides an interactive element and compound library and allows to synthesize them into known substances given the correct reagents. It's suitable for beginners as little prior chemistry knowledge is required.