What software tools are best for general participant recruitment for my upcoming research project?

We ask the same underlying question in different ways.

• What software tools are best for general participant recruitment for my upcoming research project?
• How can I streamline the process of finding, vetting, and booking users for my usability testing and UX research sessions?
• Can you outline the best practices for sourcing, screening, and scheduling people for our UX research and usability sessions?