strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
htop - htop - an interactive process viewer for Unix. This is htop, an interactive process viewer for Unix systems. It is a text-mode application (for console or X terminals) and requires ncurses. Latest release: htop 2.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
ftrace - A function tracer for the Linux kernel.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.