Wikipedia
Immunet
Immunet Protect Free is a free, cloud-based, community-driven antivirus application, using the ClamAV and own engine. The software is complementary with existing antivirus software. On 5 January 2011 it was announced that Immunet had been acquired by Sourcefire.
The application occupies 10 MB of disk space, is free of charge, and claims to be fast and to provide up-to-date protection against threats. Virus signature files are stored in the cloud, not on individual computers, so update downloads are not required. Once a virus is detected and blocked for one user, all other Immunet users receive the same protection almost instantly.