Anti-Malware

1. Ad-Aware Free 8.2
http://www.lavasoft.com/products/ad_aware_free.php
Windows
The latest version (8.2) of this venerable tool includes new tech that looks for malware patterns to ID threats, and an ultra-simple mode to automatically handle problems before you need to worry. It's free for home use; naturally, the Plus and Pro versions throw in extras, but they'll cost you ($26.95 and $39.95, respectively).
• Read PCMag's review of Ad-Aware Pro 8.1.

2. avast! Free Antivirus 5.0
http://www.avast.com/free-antivirus-download
Windows
With a new interface that makes it easier to master, as well as a new heuristic engine for finding trouble on your PC (on top of the usual definitions), this perennial freebie continues to improve. Of course, you can pay to get even more protection via avast! Internet Security.
• Read PCMag's review of avast! Free Antivirus 5.0.

3. AVG Anti-Virus Free Edition 9.0
http://free.avg.com/us-en/homepage
Windows
This app is better for keeping an already clean system clean rather than rooting out problems (according to our tests), but it excels at that, even when you're running a Windows system in Safe Mode.
• Read PCMag's review of AVG Anti-Virus Free Edition 9.0.

4. Malwarebytes' Anti-Malware 1.36
http://www.malwarebytes.org/
Windows
Even other security companies, such as like Norton, use Malwarebytes' software, probably because it's fast fast fast at scanning a system for problems. Thankfully, it also does a good job of cleaning up what it finds. Pay for the Pro version to get real-time protection.
• Read PCMag's review of Malwarebytes' Anti-Malware 1.36.

5. Microsoft Security Essentials 1.0
http://www.microsoft.com/Security_Essentials/
Windows
Microsoft realized the dirty truth: Windows is susceptible to viruses! And rather than try to make a buck off this fact, it's doing the right thing by putting out its own anti-malware for free to the masses. It's not great protection, but it's better than nothing. It works great with Windows, of course, in 74 countries and 25 languages.
• Read PCMag's review of Microsoft Security Essentials 1.0.

6. Panda Cloud Antivirus Free Edition 1.0
http://www.cloudantivirus.com/en/
Windows
This is our PCMag Editors' Choice among free anti-malware software and for good reason. Half the detection action happens in the cloud (AKA, over the Internet) so you're not downloading over-sized definition files anymore. There's no update button. It's also great at finding malware. Security expert Neil J. Rubenking called it "the best free antivirus software available."
• Read PCMag's review of Panda Cloud Antivirus Free Edition 1.0.

7. ThreatFire 4.5
http://www.threatfire.com/
Windows
It's no longer our Editors' Choice, but we still like the fact that this heuristics-only anti-malware fighter can work side-by-side with traditional definitions-based virus killers to provide supplemental protection. It's fast and even removes rootkits.
• Read PCMag's review of ThreatFire 4.5.

8. Trend Micro HouseCall 7.1
http://housecall.trendmicro.com/index.html
Windows
Like Panda, Trend is now using a cloud-based database of malware definitions to find problems, and requires no actual installation on your system. It doesn't run in the background (so no real-time protection), but that also means it's a great 'second opinion' to run along with other anti-malware programs.