sktrap (script kiddies trap) is a tiny intrusion detection system. Installed on the monitor server, it runs tests via ssh on its clients. Cracks very visible (files,open ports). Built in reply to and very succesful in finding real-world break-ins.
LiMon (Linux Monitor) is a free/open/libre multiplataform real time Linux monitor written in C/C++, and other languages. Monitor of server's relevant aspects, such as services, temperature, CPU, memory and disk usage, etc.