strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.
Treo - Treo is the page speed monitoring tool that helps your customers from obtaining a faster web experience.
ftrace - A function tracer for the Linux kernel.
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.
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
Loado - Loado is an easy-to-use monitoring tool for websites that helps developers, products, and marketers to improve user conversions and optimize SEO strategies.