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.
perf - Perf is a simple app monitoring solution paired with meaningful alerts.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
ltrace - ltrace intercepts and records dynamic library calls which are called by an executed process and the...
Better Uptime - We call you when your website goes down