Mathematics

CBB - A checkbook balancer for Unix/X11: The Check-Book Balancer - a Quicken clone CBB is a perl and tk application which can handle several bank accounts. CBB make transfers, balancing, spending analysis a breeze.
Gnumeric's user manual: The user manual for the GNOME spreadsheet. This manual covers the concepts and use of Gnumeric, the GNOME desktop environment's spreadsheet, including the formulas, the Gnumeric Solver and data analyis tools.
Gnuplot Documentation: This documentation describes the gnuplot plotting system
The GNU DC arbitrary precision calculator: GNU dc is a reverse-polish desk calculator which supports unlimited precision arithmetic. It also allows you to define and call macros. Normally DC reads from the standard input; if any command arguments are given to it, they are filenames, and DC reads and executes the contents of the files instead of reading from standard input. All normal output is to standard output; all error messages are written to standard error.