I highly recommend GSoC. It is a fantastic opportunity to learn and contribute to an open-source project by having a mentor from that project that you can talk to, and get paid to do it.

I was already contributing to an open-source project (fireflier) when I applied for GSoC for the ClamAV project, and continued to contribute to the project after GSoC was over. Shortly afterwards got invited to be part of the team, and when the project got acquired by a company the next year I got a job there to continue working on it (initially part-time because I wanted to finish my studies at the university, and after that full time).

Try to apply to 3 or 4 projects that you like, but don't get too disappointed if you don't get accepted the first time. It probably helps if you already are a contributor to an open-source project so that people can check your coding style/knowledge by looking at GitHub/Sourceforge etc.