Xv6, a simple Unix-like teaching operating system. Still Unix-like, but freshly written for teaching purposes.
gitnoc. UI for git-pandas.
OpenCorporates. Claims to be the largest open database of companies in the world.
aws-shell. An integrated shell for working with the AWS CLI.
gendo. Lightweight Slackbot framework.
TLDR pages. Interesting but needs a lot of content.
Online terminals and IDEs. Write (simple?) code online.
Elemental. A modern C++ library for distributed-memory dense and sparse-direct linear algebra and optimization.
Springer Link. Books older than 10 years old are free to download? Talk about a big turnaround. This makes Springer suddenly not all that evil after all.
How to make any plot in ggplot2?. Nice cheat sheet.