Firefox only (Windows/Mac/Linux): The VTzilla Firefox extension adds a Scan with VirusTotal option to Firefox's right-click context menu and file download dialog that allows you to scan any file for a virus before you download it.
If you're not familiar with VirusTotal, it's a brilliant web service that scans any file you send it against 42 of the best malware scanners available — and it's the tool we go to when we want to know if a download really has a virus.
You can scan files with VirusTotal by uploading them to their website, sending them via email,or uploading them via your Windows Send to menu. The VTzilla extension takes one more step out of the equation, allowing you to scan any download before you commit to downloading it to your computer. Even better: Since the latest update to VirusTotal, if the service has already scanned the file in question, the results are almost instantaneous.
VTzilla is a free download, works wherever Firefox does.