VirusTotal: uses over 40 AV engines. 20 MB file size limit.
http://www.virustotal.com/
Jotti's Malware Scan: uses 20 AV engines. 25 MB file size limit.
http://virusscan.jotti.org/en
VirSCAN: uses 37 AV engines. 20 MB file size limit.
http://virscan.org/
VirusChief: uses 13 AV engines. 10 MB file size limit.
http://www.viruschief.com/
NoVirusThanks: uses 24 AV engines. 20 MB file size limit.
http://vscan.novirusthanks.org/
Metascan Online: uses 18 AV engines. 40 MB file size limit.
http://www.metascan-online.com/
Gary's Hood: uses 5 AV engines. 10 MB file size limit.
http://www.garyshood.com/virus/
avast! Online Scanner: 16 MB file size limit.
http://onlinescan.avast.com/
Dr.Web Online Scan: unlimited
http://vms.drweb.com/online/
Kaspersky File Scanner: 1 MB file size limit.
http://www.kaspersky.com/scanforvirus
FortiGuard: 1 MB file size limit.
http://www.fortiguard.com/antivirus/virus_scanner.html