Virustotal

VirusTotal is a free online on-demand malware scanning service. You probably already have an Antivirus (AV) engine installed on your desktop so why should I use this service you ask? This service compliments your existing AV software by providing you the capability of scanning suspicious files against 36* AV engines hosted at VirusTotal. It is not a substitute for your desktop AV software.
You have the option of installing the VirusTotal Uploader software. The VirusTotal Uploader is a client software that allows you to upload individual files directly to the VirusTotal website for scanning. Once the scanning is completed, a report is displayed in your browser. This is an automated option as compared to manually uploading files to VirusTotal for analysis.

For those who do not want to install another piece of software, you can utilise the web-based option. Alternatively, you can send them files for inspection via email.

It is more likely that 36 engines would be able to detect malware. However, do note that the caveat of AV engines is that they are only able to detect what they know are malicious. 0 day exploits that they are not aware of cannot be detected by them. Watch what you download!

* 36 available engines at the moment. This number changes from time to time.