spectrwm is a minimalistic tiling window manager that tries to stay out of the way so that valuable screen real estate can be used for much more important stuff. It has sane defaults and does not require one to learn a language to do any configuration. It was written by hackers for hackers and it strives to be small, compact, and fast.
skalibs is a set of general-purpose, low-level C libraries, all in the public domain. It can replace or hide the standard C library to some extent. It is designed to allow building of small static binaries. It is used in building all skarnet.org software, including execline and s6.
Werc is a minimalistic RESTful Web application framework and content management system. It follows the Unix "tool philosophy" and it is designed to be fast, simple, convenient, and easily extensible. It handles both small and big sites and has a flexible system for user and group permissions. All data is stored in plain text files that can be easily manipulated with standard tools, without using any databases or other external dependencies. Existing applications include a blogging engine with RSS/Atom feeds, a wiki system that can easily integrate pre-existing documents (can be enabled for any directory tree), and others.
s6-portable-utils is a set of tiny general Unix utilities, often performing well-known tasks such as cut and grep, but optimized for simplicity and small size. They were designed for embedded systems and other constrained environments, but work everywhere. Other sets of small utilities are usually system-specific; for instance, the (otherwise excellent) BusyBox project only works on Linux.
getxbook is a collection of tools to download books from websites. There are tools to download from Google Books' "book preview", Amazon's "look inside the book", and Barnes and Noble's "book viewer". There is an optional GUI written in Tcl/Tk, and some shell scripts using OCR to create plain text or searchable PDFs and DjVu files from the downloaded books.
Micro Tetris is based on the 1989 International Obfuscated C Code Contest Best Game entry. It is a very small implementation, with extreme similarity to the BSD Tetris implementation. So it is very suitable for small embedded systems with only serial or SSH console access. The code uses no curses or other heavy libraries, only ANSI escape sequences and the Unix "stty" command to setup and restore the display.