OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
ftrace - A function tracer for the Linux kernel.
Monitornator - Server monitoring made simple
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.