With respect to the text below, note that FactSage is the successor of ChemSage.
If you need an interactive, standalone program that can output results in the form of tables and graphs, and can automatically calculate phase diagrams, use ChemSage/FactSage. It is an everyday desktop tool that is easy to learn and does not require programming knowledge at all.
If you need to perform equilibrium calculation from within your own or third-party programs, need to handle complex, repetitive calculations, and want a flexible programmer’s library for your projects, use ChemApp.
Needless to say, many users utilise both tools.