tcpdump is a tool for network monitoring and data acquisition.
Check_MK is a general purpose Nagios-plugin for retrieving data. It adopts a new approach for collecting data from operating systems and network components. It obsoletes NRPE, check_by_ssh, NSClient, and check_snmp and it has many benefits, the most important of which are significant reduction of CPU usage on the Nagios host and automatic inventory of items to be checked on hosts. The larger your Nagios installation is, the more helpful these improvements.
MoSShE (MOnitoring in Simple SHell Environment) is a simple, lightweight (both in size and system requirements) server monitoring package designed for secure and in-depth monitoring of a handful of typical/critical Internet systems. It supports email alerts and SLA monitoring out of the box, and whatever you can script. The system is programmed in plain (Bourne) SH, and made to be compatible with BASH and Busybox so it can easily be deployed on embedded systems. Monitoring is designed to be distributed over multiple systems, usually running locally. As no parameters are accepted from the outside, checks cannot be tampered or misused from outside. The system is designed to allow decentralized checks and evaluation as well as classical agent-based checks with centralized data accumulation. Agent data is transferred via HTTP, so available Web servers can be co-used for agent data transfer. Each agent creates simple (static) HTML pages with full and condensed status reports on each system, allowing simple local checks.
ZABBIX is software for application and network monitoring. Zabbix supports both polling and trapping techniques to collect data from monitored hosts. Flexible notification mechanism allows easy and quckly configure email notifications for pre-defined events.