I use http://www.virustotal.com/ , http://www.kaspersky.com/scanforvirus to scan files for viruses if I suspect it contains a virus.
I also occasionally use http://housecall.trendmicro.com/ , http://www.bitdefender.com/scanner/online/free.html , to scan my entire computer for viruses to give me a second opinion if there are viruses and spyware which my antivirus might of miss.
Trend Micro online scanner helped me removed a few viruses and spyware in the Windows 98 days which Avast failed to detect, but Trend Micro detected and removed.
Trend Micro, Kaspersky, and BitDefender are some of the top paid antivirus software makers in the world, so I think they won't release an ineffective online virus scanner tool for helping computer users remove viruses with their online scanners otherwise they might lose paid license software sales when their online scanner fails.
But, I wouldn't trust products made by StopSign, or some other fishy sounding company with bad user reviews or just look unsafe to use.
Indeed, they usually have ads like AVG and other free anti-viruses to try to convince you to buy their paid products when you use their browser plug-in, or add-on to scan your computer.
