Netreviver

From Macrosense:

NetReviver is a highly configurable network service monitoring application that allows any number of ports to be monitored on any number of computers, local or remote. It can be used to simply monitor any number of systems, used on each system to attempt to revive crashed server applications, or used to set up fall-over servers that become live when others fail. The program can report separate errors based on open, send data, and receive data, wrong data received, and close. When an error occurs optional e-mail notice of the error can be sent to one or two SMTP servers. Then NetReviver can execute a revival command, such as an application or a batch file in an attempt to revive the server. Optionally, another asynchronous command can be specified to execute prior to the revival command, and yet another to run synchronously with the revival command or after a specified wait period.