Even friends can unknowingly send us viruses, especially if a link comes from “source unknown”, so we've been erring on the side of caution using a link security service which has given us an extra piece of mind before opening www.mostquestionableURLs.com: VirusTotal.com.
This website is free and user friendly. On the Analysis section of the their website you can choose to upload a file to be checked or submit a url. It will scan the url and detect any viruses or malware, and then issue you with a report.email interface, which permits users to upload files via email and services up results of the scan back into your mailbox. All you have to do is send an email to email@example.com with SCAN (for a report in plain text) or SCAN+XML (for a report in XML) in the subject field. Attach the file to be analyzed (any files over 20MB will be rejected).
Perhaps overly cautionary, but a useful (and did we mention free) tool for the occasional email that riles up any of your suspicious content 6th sense.