VirusTotal is an excellent free web service which scans any file for malware against 50+ of the top antivirus engines. But it can take a moment to use, as you must visit the site, choose the type of scan you need (file or link), then point VirusTotal in the right direction.
VTzilla and VTchromizer are Firefox and Chrome add-ons which simplify this process by integrating VirusTotal scanning options into the browser, enabling you to check any file or URL with one or two clicks.
A normally safe site might occasionally host something dangerous, of course, and both VTzilla and VTchromizer add an addition “Scan with VirusTotal” option to the right-click menu for a download.
VTzilla then goes one step further by adding a “Scan with VirusTotal” button to the Firefox download box. This is good, as it acts as a permanent reminder to be careful about what you’re downloading, and enables you to run a scan with just one click. But it’s also more intrusive, and may not appeal to everyone.
VTzilla takes a similar, more “in your face” approach elsewhere. In particular it adds a toolbar where you can search VirusTotal for hashes, URLs and more, or launch a scan of your current site; this works, but it takes a lot of screen real estate and isn’t customizable (although you can at least turn it off). Meanwhile VTchromizer is more subtle. It only adds a single button to the Chrome toolbar, but this still gives access to a VirusTotal search box and a “Scan current site” link.