Student developers looking to work on the popular LLVM compiler infrastructure (or one of its sub-projects like the Clang C/C++ compiler front-end) this summer while being paid for their contributions and learning how to engage in open-source development, LLVM is once again participating in Google's Summer of Code.
There's a long list of open projects for interested students. There's work ranging from adding benchmarks and performance testing to LLVM to adding new capabilities and working on features like Profile-Guided Optimizations. Code compaction support to optimize for code size is another possible feature being sought after.
On a related note, the LLVM Linux project is also seeking GSoC attention. Developers are still hard at work on making the upstream Linux kernel compatible with building under LLVM/Clang rather than just GCC. Much progress has been made in being able to build the Linux kernel with Clang but there's still outstanding patches, etc.