List of software

Downloadable Antiviruses
Please note that ALL of these antiviruses have free and paid versions. The only difference being paid versions usually have 1 on 1 tech support via web or phone, while free versions are restricted to forum assistance. If you wish to use the free version, disregard any popups (ie AVG) about purchasing the program, and get an updated free license.
 * Avast Free Antivirus
 * Avira Free Antivirus
 * Security Essentials

Online Scanners

 * HouseCall Scanner

Anti-Malware

 * Malwarebytes

Rootkit Scanner

 * GMER (Note AVAST includes GMER in its free antivirus)
 * Sysinternals
 * Anti-Rootkit

Special cases

 * ComboFix - Use with care