The guys at CastleCops Wiki have ransacked the net to produce a complete list of online scanners covering almost every major antivirus/trojan/spyware vendor. Full disk scanners (26), single file scanners (11), multiple engine scans (4) sandboxing scans (5) and misc. Includes details like screenshot, file upload limits, engine used, cleaning etc

http://wiki.castlecops.com/Online_malware_scans_-_Comparison

"Below are a list of scanners that scan your full hard-disk. The engine underlying each scan is noted so you can avoid duplication with scanners you already have. If a freeware version of the scanner (or at least roughly the same engine) is available for local use it is indicated. Most allow you to scan specific folders as well. Almost all have options to scan archives, those that do not are "tested" by scanning with a zipped version of eicar, though this is not a foolproof test. A few scanners only detect problems but do not offer to disinfect or even delete the offending file; these are indicated."