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).