I know some of these have been posted before but I figured it would be helpful to have all these links in one place.

Scan a single file with multiple engines:

http://www.virustotal.com
http://virusscan.jotti.org

Scan your whole computer:

http://housecall.trendmicro.com/
http://www.pandasoftware.com/products/ActiveScan.htm
http://support.f-secure.com/enu/home/ols.shtml
http://www3.ca.com/securityadvisor/virusinfo/scan.aspx
http://us.mcafee.com/root/mfs/scan.asp?affid=56
http://www.bitdefender.com/scan8/ie.html
http://security.symantec.com/sscv6/home.asp?langid=ie

I am sure those are not all of them but those are some very popular ones. Enjoy :-)