AVG Free was updated for Window 8 already back in September. With the official release and general availability of Windows 8 this week, Avast also updated their free antivirus to support the newest Microsoft operating system. Update: Seems Avast already was ready for Windows 8 when they released version 7 back in February 2012.
avast! 7 is Windows 8 Beta compatible and includes specific features for Windows 8 such as the Early-Load Antimalware Driver (ELAM) against system-level rootkits and bootkits.
Another less known alternative is Panda Cloud Antivirus, which also supports Windows 8. Version 2.0 was released earlier this year and has done well in tests performed by AV-Comparatives.org. In September 2012 Panda Cloud AV achieved Virus Bulletin VB100 award.
Windows 8 already includes an anti malware solution called Windows Defender, which is actually just Microsoft Security Essentials re-branded. The Windows Defender name debuted six years ago, then only as an anti spyware product with some additional features.