advise for Open Source project to gather experience

Hi Guys, I have been away for programming for while as my current job is in different field in wireless telecommunications,but I would like to change my carrier path and start as a developer. I have scjp 5.0. I want to gather experience by contributing to open source project. Please suggest me, which open source project is popular or valuable for my resume. Also , if any one of you worked on any open source project that you liked and share the experience. Thanks a bunch in advance

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