strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.
Website Grader - Grade your website's strengths and weaknesses in seconds.
ftrace - A function tracer for the Linux kernel.
PageSpeed Ninja - One-click page speed boost for WordPress
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
WebSite Auditor - The techie's SEO spider: find site issues other tools miss