Frunk is a simple program that detects and reports interface status changes using SNMP. Basically, it detects if one or more of a set of pre-configured interfaces are up or down and reports the fact by emailing alerts. It is good for monitoring several interfaces on several devices, normally routers, and checking for link failures. It also optionally creates and constantly updates an HTML document containing information on those interfaces which are down.
OpenSSG is an implementation of the Cisco Service Selection Gateway, which is available on IOS devices with the required feature image. It uses libipq for user-space packet inspection. Traffic flows of unauthenticated users are redirected to a SESM (Subscriber Edge Service Manager), where they are authenticated by whatever means (an HTTP server with a database backend, or a telnet server). Upon authentication, the user is authenticated with a certain IP and this is sent to the SSG server.