This website is www.virustotal.com. VirusTotal is a service that analyzes suspicious files and facilitates the quick detection of viruses, worms, trojans, and all kinds of malware detected by antivirus engines. This free, independent service features the use of multiple antivirus engines (Which is very important to be able to capture the newest viruses as quickly as possible) and real-time automatic updates of virus signatures. additionally, it also provides detailed results from each antivirus engine.
For those who are reluctant to install an anti virus software on their system (bad idea!) or maybe for those who just want to take the extra step to scan certain suspicious files for any infections, this could be the service you are looking for!