ZABBIX is an enterprise-class distributed monitoring solution designed to monitor and track performance and availability of network servers, devices, and other IT resources. It supports distributed and Web-based monitoring, auto-discovery, real-time monitoring, SLA assurance, trending, and more.
di is a disk information utility that displays everything that df does and more. It features the ability to display your disk usage in whatever format you prefer. It also checks the user and group quotas, so that the user sees the space available for their use, not the system wide disk space. It is designed to be highly portable across many platforms and is great for heterogenous networks.
Spacewalk is a Linux and Solaris systems management solution. It allows you to inventory your systems (hardware and software information), install and update software on your systems, collect and distribute your custom software packages into manageable groups, provision (Kickstart) your systems, manage and deploy configuration files to your systems, monitor your systems, provision virtual guests, and start/stop/configure virtual guests.
Pandora is a distributed system to monitor processes, performance, status, application, or operating parameters of almost any operating system. It has a decentralized management system, based on flexible user profiles, that allows the user to generate graphical reports, define alarms, and a full incident management system to operate a 24x7 monitoring team.
net-status-monitor is a simple console script to test whether Internet connectivity is working. If it is, the script tells you the external IP address (in case your dynamic DNS provider is down). If not, it tries to find the point where the connectivity fails. The output is quite simple, so you can put it on your grandmother's PC. "Just tell me what the read line says" should give you enough information.
Monjo is a Web application that helps monitoring a small to medium network of servers. It offers easy customization of notifications in case of network failures and is designed to allow for easy addition of new services that need to be monitored. It's internationalized and includes a Web interface in English and German. It currently supports notifications by email and can check services using ping, SMTP, HTTP, POP3, and IMAP4.