VirusTotal
virustotal.com
Upload any file or paste any URL and VirusTotal scans it simultaneously against over 70 antivirus engines and security vendors, returning a detailed report on whether anything flagged it as malicious.
Why it is useful
A second opinion that actually matters — if one antivirus misses something, 70 others might catch it. Essential before opening a downloaded file you are not sure about, or before clicking a link sent in an email. Also lets you scan a URL without visiting it, check a file hash to see if it has been analysed before, and view community comments on suspicious files.