Jmol is a Free, Open Source molecule viewer and editor. It is a collaboratively developed visualization and measurement tool for chemical scientists. Jmol is an active project, and there are new features being added to it on a daily basis. Users are encouraged to modify it to fit their needs and to contribute their changes to the project.
JOELib/JOELib2 is a cheminformatics library which supports SMARTS substructure search, descriptor calculation, processing/filtering pipes, and conversion of different chemical file formats. It is written in 100% pure Java, and interfaces to external programs (e.g. Ghemical) are available.
ProteomeCommons.org Java Analysis Framework is a framework for referencing common mass spectrometry data such as atoms and residues. This framework provides a programmer with an easy-to-use library for writing programs that rely on common mass spectrometry data. The framework also contains utilities such as an isotope calculator and a reference for residue abbreviations.