xombrero is a minimalist's Web browser. It strives to be vi-like and is suited for heavy keyboard users.
mcwm is a minimalistic window manager implemented using XCB. It doesn't use libX11 at all. It is a traditional floating window manager with no fancy smarts like those in modern tiling window managers.
pyme is a minimalistic GUI editor. Its feature list is intentionally limited to creating, opening, editing, and saving files.
U.D.O.S. zero is a minimalistic C, ASCII, VRML, non-privileged User Distributed Operating System.
Software for handling mailing lists, with a nice user-oriented Web interface.
A query processing engine that defends against inference problems.