Somethings to dig your security teeth into: ACL support for the Linux kernel.
Access Control Lists allow fine grained access control to filesystem objects, by attaching a list of permissions to grant or deny specific capabilities to users or groups.
This implementation of ACL for the Linux kernel provides semantics that are almost totally compatible with the traditional POSIX umode model for applications that are unaware of the kernel support.
Features include the ability to set rights for fine grained operation on filesystem objects (such as separate write/truncate/append permissions) to an arbitary number of users or groups; and the ability to "offer" a file for chown()ing by another user.
Currently, using the package requires patching and recompiling your kernel, and installing tools to use the new features, thus requiring some kernel-fu savvy.
Once development has reached a stable, reliable state and has been well tested, the kernel patch aspect will be submitted for inclusion in the main kernel sources.
Testers are welcome, and peer review of the security aspects of the code are welcome, and desired.