The Engineering ToolBox is a website that provides free access to a massive collection of technical resources, tools, and basic information for engineering and design. It covers a wide range of disciplines, including mechanical, electrical, chemical, civil, and environmental engineering.
(Biggest issue)
Cross-check any critical number from Engineering Toolbox against a primary source (NIST, ASME, textbook, manufacturer datasheet). Treat it as the engineering equivalent of Wikipedia – wonderfully useful, but never the sole authority.