PortSentry

A very good tool to detect and respond to port scans against a target host in
real-time. It runs on TCP and UDP sockets and works on most *nix
systems. Advanced stealth detection modes are available under linux only, and
detect SYN, FIN, NULL, XMAS, and Oddball packet scans. All modes support
real-time blocking and reporting of violations. This also interfaces very well
with ipchains firewalling rules, allowing a workstation to lock out a potential
hacker from the first bad port that he probes.

http://www.psionic.com/abacus/portsentry