strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
ftrace - A function tracer for the Linux kernel.
UptimeRobot - Free Website Uptime Monitoring
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
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.