Sting is a simple, host-based approach to detecting arp cache poisoning based man in the middle attacks (such as made by ettercap) on your LAN. It uses SNMP to periodically query the arp cache of your router and make sure its entry for you is correct.
GPP is a General Purpose Proxy Java graphical application intended mainly for packet inspection and modification. It's main idea is to be a little user-friendly portable man-in-the-middle tool for security analysis. Later, some protocols should be added