Open Source
I am a huge Open Source fan. There is virtually no project in the world that doesn’t use it. Java? Open source. Spring? Open source. Javascript, HTML, CSS3, AngularJS? Open source. JPA, Hibernate? Open source. The library you sent emails with? Open source! You get the idea.
During my career and my hobbies, I often write some library or plugin to help me with my technical challenges. Often I will make these Open Source so others can benefit from it.
You can find my more recent contributions on Github.
Ofcourse, every developer uses Stackoverflow these days and I contribute there as well by answering some questions every now and then in addition to some moderating tasks I perform there.