Netcraft is a tool that provides internet security services, including the detection of phishing and spam emails. It offers a range of services that can help organizations identify fraudulent websites and phishing activities by analyzing web content and email messages for known phishing signatures and heuristics. This makes it a useful tool for incident handlers like Francis, who is tasked with detecting phishing and spam emails for client organizations. Other options listed, such as Nessus (a vulnerability scanner), BTCrack (a Bluetooth pin and link-key cracker), and Cain and Abel (a password recovery tool), do not specialize in detecting phishing or spam emails but serve different purposes in cybersecurity.