This is the project page for a Layer 7 (application layer) packet filter for Linux. Unlike traditional packet filters (which is usally based on ports, source and destination addresses, etc.), this can distinguish between different types of data on an application level. Which means it can, for example, distinguish regular HTTP traffic from P2P app traffic also using port 80.
I messed with a standalone device that does this a few months back. It was quite powerful and capable, and I'd like to see this kind of ability available to Linux. This project is currently at a 0.3 release, but it should be worth keeping an eye on it.