Here's some (totally free) stuff you should definitely check out!
- git - Distributed version control system (http://git-scm.com).
- zsh - Powerful command line interpreter (http://www.zsh.org).
- vim - Modal, highly-configurable text editor (http://www.vim.org).
- LaTeX - Document preparation system (http://www.latex-project.org)
- Lua - Lightweight embeddable scripting language (http://www.lua.org).
- MINIX - Reliable microkernel-based operating system (http://www.minix3.org).