Honeyd is a small daemon that creates virtual hosts on a network. The
hosts can be configured to run arbitrary services, and their TCP
personality can be adapted so that they appear to be running certain
versions of operating systems. Honeyd enables a single host to claim
multiple addresses - I have tested up to 65536 - on a LAN for network
simulation.

WWW: http://www.citi.umich.edu/u/provos/honeyd/