Windows only: Previously mentioned VirusTotal Uploader automatically uploads any file to online virus scanner VirusTotal, scanning it for viruses with 41 different popular antivirus applications — and now it's even better and faster, with instant hash checking, letting you skip uploads altogether.
Once you've installed the tiny VirusTotal Uploader application, you can simply right-click on a file and use the Send To -> VirusTotal option, which will take a hash of the file (a unique fingerprint that identifies the file) and submit it to the VirusTotal service. If the file has already been scanned by VirusTotal, you will see a message saying that the hash was found, and your default browser will be opened to the scan results instantaneously. If the file hasn't already been scanned by VirusTotal, it'll continue uploading the file; you can also choose to re-upload an already-scanned file if you wish.
You can also now open the VirusTotal Uploader window directly, choose from running processes, upload multiple files, or even download a file from a URL and automatically upload it to VirusTotal (without storing the file on your PC).
It's a great update to an already excellent way to find out if a file really has a virus. VirusTotal Uploader is a free download for Windows only.