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. ↪
No questions with score of 5 or more