Free Online Tools for Looking up Potentially Malicious Websites

Several organizations offer free online tools for looking up a potentially malicious website. Some of these tools provide historical information; others examine the URL in real time to identify threats:

Any on-line tools that should be on this list, but are missing? Let me know. Visit other lists of on-line security resources outline Automated Malware Analysis Services and Blocklists of Suspected Malicious IPs and URLs.