The Bubbling Load Monitor (or "Bubblemon" for short) is a system load monitor for the GNOME panel. It looks like a vial containing water. The water level indicates how much (electronic) memory is in use. The color of the liquid indicates how much swap space is used. The amount of bubbles reflects the system CPU load. A message in a bottle indicates there is unread mail. A reed-like graph shows I/O load. On multi-core systems the CPU with the highest load will bubble in the middle, and the others on the sides, so it's possible to see how well load gets distributed between CPUs.
Cheops-ng is a network management tool for mapping and monitoring your network. It has host/network discovery functionality and OS detection, and does a port scan of each computer to tell what services are running. It can probe FTP, HTTP, ssh, email (port 25), news, IMAP, and VNC to see what version and type of server is running on those ports.
Firestarter is a firewall tool for Linux, and uses GNOME. You can use the wizard to create a basic firewall, then streamline it further using the dynamic rules. You can open and close ports with a few clicks, or stealth your services giving access only to a select few. It features a real-time hit monitor which you can watch as attackers probe your machine for open ports.
gbox_applet is a GNOME applet which monitors several mbox-type mailbox files. Each mailbox can be assigned to a priority between 0 and 2 and it can have its own time interval for checking. For each priority there is a different icon. gbox can also count the mail in the boxes and show the subject/sender of each mail in the applet's menu. A tooltip shows a summary of the information in three different kinds.