These all download a small program that then runs their virus scan on your machine:
BitDefender Free Online Virus Scan
McAfee FreeScan
RAV AntivVirus Online Virus Scan
TrendMicro HouseCall
This one is a little different. You tell it which file to check and it automatically uploads the file to a dedicated server which then runs a scan on it to see if a virus is present:
Kaspersky Lab Online Virus Scanner