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:

• AbuseIPDB: Provides reputation data about the IP address or hostname
• Auth0 Signals: Checks IP address reputation; supports API
• BrightCloud URL/IP Lookup: Presents historical reputation data about the website
• CheckPhish: Checks whether the URL is a fraudulent site
• CyberGordon: Look up the website (and other observables) across several services
• Desenmascara.me: Flags websites suspected of selling counterfeit products
• Email Blocklist Checker: Checks the domain name or IP address against email blocklists (email address required, opts into marketing).
• FileScan.io: Examines the URL in real time
• FortiGuard lookup: Displays the URL's history and category
• Google Safe Browsing: Look up the website's current status
• hashdd: Provides historical data about IPs, URLs, etc.
• IBM X-Force Exchange: Provides historical data about IPs, URLs, etc.
• IPQualityScore: Presents a risk ranking for the IP address
• Joe Sandbox URL Analyzer: Examines the URL in real time
• Ironscales Fake Login URL Scanner: Examines the URL for signs of phishing
• Is It Hacked: Performs several checks in real time and consults some blacklists
• IsItPhishing: Assesses the specified URL in real-time
• Kaspersky Threat Intel Portal: Looks up the IP, URL, or domain in a blacklist
• Norton Safe Web: Presents historical reputation data about the website
• Palo Alto Networks URL Filtering: Looks up the URL in a blacklist
• PhishTank: Looks up the URL in its database of known phishing websites
• PolySwarm: Uses several services to examine the website or look up the URL
• Malware Domain List: Looks up recently-reported malicious websites
• MalwareURL: Looks up the URL in its historical list of malicious websites
• McAfee Site Lookup: Checks URL reputation in various McAfee lists
• MxToolbox: Queries multiple reputational sources for information about the IP or domain
• Open Threat Exchange: Presents diverse threat intelligence data from AlienVault
• PassiveTotal: Presents passive DNS and other threat intelligence data
• Pulsedive: Presents historical data and queries for additional information
• Quttera ThreatSign: Scans the specified URL for the presence of malware
• Scamadviser: Checks whether the website is likely a shopping scam
• SecurityTrails: Provides current and historical domain or system data
• Sucuri SiteCheck: Scans the URL for malware in real-time and looks it up in several blacklists
• Talos Reputation Lookup: Presents historical reputation data about the website
• Trend Micro Site Safety Center: Presents historical reputation data about the website
• ThreatSTOP Check IoC: Looks up the UP or domain in a blacklist (requires your email address)
• urlscan.io: Examines the URL in real time and displays the requests it issues to render the page
• URLVoid and IPVoid: Looks up the URL or IP across several services
• VirusTotal: Looks up the URL in several databases of malicious sites
• ThreatMiner: Presents diverse threat intelligence data
• WebPulse Site Review: Looks up the website in BlueCoat's database
• Zscaler Zulu URL Risk Analyzer: Examines the URL using real-time and historical techniques
• zveloLive: Looks up the website in its database of categories

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