AysMon for Mac

From Pepsan & Associates:

AysMon (Are You Serving Monitor) is a multi-purpose utility for monitoring system resources associated with network services. System administrators and other expert users benefit from continuous monitoring of critical aspects of network services of the host system. End-users will find AysMon an effective way to monitor throughput on their DSL, Cable, PPP, or other network connections as well as providing a friendly display of disk space usage and key open network ports. The latter is important since you may have a network service running that unintentionally opens your system to attack by hackers. AysMon enables to see at a glance if this is the case.

An important benefit of AysMon during the transition to Mac OS X is that these capabilities are available to you without having to learn anything about a command line interface or the Unix underpinnings of Mac OS X. While expert users may wish to run AysMon continuously, end-users may prefer to run it periodically to check their system's configuration or to monitor network throughput during troubleshooting or just out of curiousity.

AysMon is Java-based and self-configures through the use of auto-discovery of available disks (any media type) and network interfaces. Just double-click the application and it will sense your system's configuration and display the relevant information. You can also customize AysMon to your needs using preferences to control which services to monitor (including HTTP, FTP, DNS, Telnet, Finger, etc.) and how often to update the display of each window. AysMon remembers window positions and sizes so you can put them exactly where you want.