When I'm not busy with working, sleeping, or eating, I spend my time contributing to Open Source software. One of my recent favourites are certain Mozilla projects.
Mozilla is the non-profit behind Firefox. They also have their own smartphone and mobile Operating System, FirefoxOS. I got one of the phones at Mozfest, and ever since I have been working on apps for it, both system apps and third party apps. Because all the apps are developed in HTML5 and JavaScript, they are much easier to contribute to than other technologies (e.g. IOS or Android), at least for me. Therefore, it's a great experience to contribute to.
One of the things I have been working on has been implementing user-defined playlists to the music app.. I thought (and I think I was right!) that it would be a big challenge, but a good and achievable one.
I was surprised at how quickly I discovered how the music app (which was code I had never looked at before) ties together with its own components as well as parts of FirefoxOS and how everything works. I haven't finished the feature yet, but I'm slowly getting there.
Another project I contribute to is webmaker. It's an amazing initiative to help new Internet users be Makers instead of just Consumers, by enabling them to learn how to create and edit websites and mobile apps. I got involved with that project through Social Coding 4 Good and have been fixing big and small bugs, implementing new features (such as caching with appcache), and taking part in demos and meetings. I love the project and its cause, and learning the technologies used in the project (node and angular) is useful work experience, so I expect to be contributing for a long time to come!