OREKIT is a free low level space dynamics library written in Java. It provides basic elements (orbits, dates, attitude, frames, ...) and various algorithms to handle them (conversions, analytical and numerical propagation, pointing, ...).
It the FAQ it is mentioned that the Two Line Element (TLE) package is well validated, even some errors in the original Fortran code from NORAD were found.

yEd is a powerful desktop application that can be used to quickly and effectively generate high-quality diagrams.
Create diagrams manually, or import your external data for analysis. Our automatic layout algorithms arrange even large data sets with just the press of a button.

yEd is freely available and runs on all major platforms: Windows, Unix/Linux, and Mac OS X.

Zero Install is a decentralised cross-distribution software installation system. Other features include full support for shared libraries (with a SAT solver for dependency resolution), sharing between users, and integration with native platform package managers. It supports both binary and source packages, and works on Linux, OS X, Unix and Windows systems. It is fully Open Source.

A port of BSD2.11 to a MIPS-based microcontroller with 128 kiB RAM.

MXE (M cross environment) is a Makefile that compiles a cross compiler and cross compiles many free libraries such as SDL and Qt. Thus, it provides a nice cross compiling environment for various target platforms (including Windows).

This is a distribution package of ancient UNIX 7th Edition together with a PDP-11 simulator and instructions how to get things running.

New (started in 2012 TBC) open source (GPL) editor supporting Clojure, ClojureScript, Javascript, Python, HTML, and CSS. More languages can be added via plugins in the future. LightTable is written in ClojureScript and runs on Windows, OS X and Linux.

Program to query and set the status of the keyboard LEDs on Mac computers.

The RADclock project (formerly known under 'TSCclock') aims to provide a new system for network timing within two years. We are developing replacements for NTP clients and servers based on new principles, in particular the need to distinguish between difference clocks and absolute clocks. The term RADclock, 'Robust Absolute and Difference Clock', stems from this. The RADclock difference clock, for example, can measure RTTs to under a microsecond, even if connectively to the time server is lost for over a week!
The RADclock is available and can be used as an alternative to the ntpd daemon to provide convenient system clock synchronisation for systems running FreeBSD and Linux. In terms of local hardware, it exploits the availability of counters such as the HPET counter or the TSC (Time-Stamp-Counter) commonly found in PC architectures.

The sigrok project aims at creating a portable, cross-platform, Free/Libre/Open-Source signal analysis software suite that supports various device types (e.g. logic analyzers, oscilloscopes, and many more).

It is licensed under the terms of the GNU GPL, version 3 or later.