Not tried this myself yet, but it looks interesting:
And here's the autoupdate script:
Requires that you have ClamWin installed of course:
SourceForge.net: ClamWin Free Antivirus
Hmm, looks like it's worth testing, there was a project a while back that integrated ClamAV to get on access scans but I think it's long abandoned now.
Last time I checked the detection rate on ClamAV it was about 15% lower than most commercial AVs - is that still the case?
No, it's about the same for new virus signatures. For older viruses, they add signatures when time permits, so detection there can be a little weak.