I would like to practice specific technologies/frameworks like Hibernate, Spring, GWT and so on. How can I find an open-source project that uses specific frameworks technologies so I can contribute to it?
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There are a number of ways, my preferred one is to use Github, as many OSS projects are hosted there:
Often, big open source projects have a "get involved" page that lists IRC channels, projects that use the software, etc:
Basically, join the specific community by going to their IRC channels, GitHub projects, etc. Once you know the technologies' communities, you'll know of other projects you can contribute to.