Here's some (totally free) stuff you should definitely check out!

• git - Distributed version control system. ↪
• zsh - Powerful command line interpreter. ↪
• vim - Modal, highly-configurable text editor. ↪
• latex - Document preparation system. ↪
• lua - Lightweight embeddable scripting language. ↪
• minix - Reliable microkernel-based operating system. ↪