Sage is a free mathematics software system licensed under the GPL. It combines the power of many existing open-source packages into a common Python-based interface. Mission: Creating a viable free open source alternative to Magma, Maple, Mathematica and Matlab.
Compression tool similar to gzip and bzip2, with better compression and speed than bzip2 at the cost of higher memory consumption (mostly for compression). From a GNU coreutils announcement.
Plot Digitizer is a Java program used to digitize scanned plots of functional data. Written in Java. autotracing is supported on Mac OSX, Windows and Linux. From MP.
A toolkit to work on PDF documents. From the page: "If PDF is electronic paper, then pdftk is an electronic staple-remover, hole-punch, binder, secret-decoder-ring, and X-Ray-glasses."
Cool page about (semi)numerical algorithm, including a book in progress, an open source library and a number of interesting links. Also has a mirror of hakmem. The author has also published a book about pi.
The home page of the SoftGPS receiver which accompanies the book A Software-Defined GPS and Galileo Receiver A Single-Frequency Approach by K. Borre et al. (used to be on http://gps.aau.dk/softgps/ )
C/C++ version of the orbit models used with TLE data.
Source code for astronomical calculations, including satellite orbits.
See also https://github.com/Bill-Gray/sat_code/ which is licensed under the GPL V2.