Trailblazer is a high-level Architecture libraries for the Ruby web application (not only for Rails). If you are no familiar with it yet - take a 20 minutes walk through guide...
I want to cover once again those places where our team had issues or misunderstanding. Trailblazer documentation got a lot of improvements recently and keep getting more and more care. Some points from this list are covered by the docs, but in practice not everything was smooth.
Sysdig - open source, system-level exploration: capture system state and activity from a running Linux instance, then save, filter and analyze. Kind of strace + tcpdump + lsof + awesome sauce. With a little Lua cherry on top.
LetsEncrypt.org promises till Summer 2015 to provide free Certification Autority to make https for our sites just as with a magic wand. That will be awesome :)
Vivus - JavaScript library to make drawing animation on SVG
Google's Material Design like CSS frameworks
Factory Muffin - library for creating and populating objects (with Faker) on-the-fly in tests. Also could be used with ORM to poluplate DB objects.