I often talk to students that want to contribute to open-source projects, but just don't have an idea what to work on. Here's a tip if you're in a similar situation (e.g. you want to apply for GSOC) :

1 git clone repository_url_of_some_open_source_project target_directory
2 grep -RIn TODO target_directory/*

So, find the URL of the repository project you want to contribute to, checkout the repository using git/mercurial/svn and then find all the TODOs in the source code using grep.