Smit is an ARP-level transproxy/sniffer, capable of relaying local traffic even in switched environment. The source is based on the arprelay and arpmitm tools, but offers a nice command line interface and option of saving raw packets into a file. It includes some tricky techniques for ARP-cache consistence. You can also run Smit in transproxy-only mode and use your favourite sniffer to capture 'hijacked' packets, and it includes a ring buffer option for high performance packet capture.
