Darkstat is a packet sniffer that runs as a background process on a cable/DSL router, gathers all sorts of statistics about network usage, and serves them over HTTP.
* Traffic graphs, and reports per host, showing each port for each host.
* Embedded web-server with deflate compression.
* Asynchronous reverse DNS resolution using a child process.
* Small. Portable. Single-threaded. Efficient. Uncomplicated.