The AbiWord word processor is a full-featured cross-platform word processor.
AquaTerm is a Mac OS X grahics renderer. It allows command line applications written in ObjC, C, FORTRAN, Lisp, Perl or Python to display vector graphics, text and images using a simple API. Adapters for gnuplot, PGPLOT, and PLplot exists as well.
The Mac TeX Toolbox provides an umbrella for TeX related development for Mac OS X: discussions, pointers to libraries and other support for open source TeX development on Mac OS X.
The Open Descent Foundation (ODF) is a community for developers, who want to help to create open source programs, tools, etc. for the famous games Descent1, -2 or -3. Everyone can join the ODF if he accepts the idea of writing open source programs.
The Gnuritas project aims at designing a "smart" and flexible framework and tools to manage and share knowledge, information and scientific data.