Having and using an up to date virus/malware scanner is essential these days. Below is a list of free resources to ensure that you remain safe even if you can't afford to pay for a subscription.

Windows:
AVG Free - Realtime and on-demand
TrendMicro HouseCall - Web-based on-demand
McAfee Security Scan - Realtime and on-demand
Kaspersky Virus Removal Tool - On-demand
Microsoft Safety Scanner or Security Essentials - On-demand
Norton Security Scan - Realtime and on-demand
Avira Free Antivirus - Realtime and on-demand
BitDefender QuickScan - Web based on-demand (targeting a subset of web-based threats only)
Avast Free Antivirus - Realtime and on-demand

In addition to using a scanner above, Windows users would probably also like to install the following:
Malware Bytes Anti-Malware Free - Specialised targeting for certain difficult malware
Spybot Search and Destroy - Spyware remover
Ad-Aware Free - Spyware remover
Microsoft Malicious Software Removal Tool - Specialised targeting for a very limited subset of malware

Mac:
Sophos Antivirus for Mac Home Edition - Realtime and on-demand
Avast Free Antivirus for Mac - Realtime and on-demand
Avira Free - Realtime and on-demand


Linux/Unix:
AVG Free
Avira Free Antivirus