Summer of Code is a great program - providing stipends to students and more importantly connecting them with mentors in open source projects. ZooKeeper has a number of interesting projects to get started on.
ZooKeeper is a distributed coordination platform on which you can build the distributed equivalents of many traditional concurrent primitives like locks, queues and barriers. It’s heavily used in the real world - Yahoo! use it extensively, and many other major companies rely on it. The other committers and I are actively looking to increase participation in the project - there is loads of really interesting work left to do. If consensus protocols, distributed systems, scalability, fault-tolerance and performance are your thing, this is certainly the project for you.
If you have any questions at all, drop me a line at henry at apache dot org.