Qualys SSL Labs – Website SSL testing suite to determine a sites configuration and if improvements can be made. All sites should be aiming for an A+ here.
Pen-Test Tools – A suite of tools for users to perform online pen-testing. Quite comprehensive but users can use their credits up quick.
MX Toolbox – Mail server tool box that can help determine issues that users are having with mail servers or check the health of a server after configuration. Essential for ensuing your mail server isn’t a public relay before going online.