I've contributed to a lot of open source projects - many of which are just one off contributions/fixes. Projects listed here are projects I'm actively involved with and built major parts of. To see everything you might want to check out my github profile.
benchee is a benchmarking library for elixir that provides a wealth of statistical data and is very extensible. The most prominent extension point is the ability to use different formatters for CSV, JSON and HTML output.
Among others benchee also overs hooks, parallel execution, recording of previous results and much more. It's the standard benchmarking solution in the Elixir eco system.
SimpleCov is the default code coverage library in the Ruby ecosystem. It provides both line coverage and branch coverage, generating easy to read HTML reports as well as allowing you to write your own formatters.
Shoes is a graphical toolkit for ruby - i.e. it enables you to write desktop applications with ruby. It features a beautiful DSL that allows you to easily create these applications. One of its main applications in my eyes is teaching beginners and especially kids how to program.
It's the first open source project I ever contributed to. Mostly working on a rewrite called shoes4.
after_do is a tiny library to support Aspect Oriented Programming like techniques in Ruby. E.g. you can specify blocks to be executed after methods of specific classes are called. I wrote an introduction blog post.
The Rails Beginner Cheat Sheet is just what it sounds like - a simple cheat sheet for people starting to learn rails. Through my coaching at Rails Girls Berlin and in the project group I noticed that there are several questions being asked over and over again and certain commands that are hard to remember. So I went looking for cheat sheets but returned unsatisfied. Most cheat sheets were not made for beginners, they lacked explanations and were far too technical.
I like to go to numerous community events, be it user groups, conferences or just a get together. Communities listed here, are the ones I'm actively involved with.
I organize the Ruby User Group Berlin and have been doing so since ~November 2012. This entails finding a location/host, picking talks, keeping the website up to date, moderation etc. And of course it entails a lot of fun. For me RUG-Bs are always too short as I want to talk to everyone but never can.
I'm a regular coach at Rails Girls Berlin events, such as beginner/advanced workshops and hackdays, since summer 2012. Moreover I coach a weekly project group, the rubycorns, together with Til since march 2013 and we develop rorganize.it.