Free File Upload Sites for Virus Scanning

If you have a file you suspect may be infected by malware or simply want to check a file to ensure that opening or running it, if it is an executable file, won't infect your system with a virus, trojan, worm, adware, or spyware, there are sites where you can upload the file for immediate analysis.

Online Scanning Sites with Multiple Scanners
SiteComments
VirusTotal As of April 5, 2009, VirusTotal will scan a file with 40 malware detection programs. For a list of the antivirus engines it uses see About VirusTotal. You can also send files to be scanned by email; see Sending files by email. Note that there's a 10MB file size limit, and if you forward a message with multiple attachments, VirusTotal will only check one.
VirSCAN Scans a file with 37 malware detection programs as of April 5, 2009. There is a 10 MB file size limit per file.
Jotti This is a non-profit service that will scan the uploaded file with 20 antivirus programs as of April 5, 2009. There is a 10 MB file size limit per file.

Individual Scanners

Dr.Web - click on the browse button and then the scan button when you've selected a file.

There are also other sites where you can opt to have your entire system scanned for malware (see Free Online Virus Scanners)