I've listed five which are available now as free betas (you may have to pay for the shipping versions, but you can download, install, and play with most of these now). The list comes via the keen posting eye of the moderator of the TechNet Vista security forum. I've slightly respun it with updated info, where necessary. (His original post is here.)
PC-cillin from Trend Micro. A beta version of PC-cillin 15.3 can be downloaded for free.
Windows Live OneCare. Free 90-day trial here.
CA, the former Computer Associates. Free CA Anti-Virus Beta For Vista here.
McAfee Total Protection For Small Business. Free beta of what's billed as an "integrated security software as a service -- providing virus, spyware, firewall, and now browser protection, as well as centralized management."
F-Secure Anti-Virus Beta For Vista. They call their free download the "7.0 Beta".
As for leading security vendor Symantec, it doesn't seem to be offering any free downloads. Their Vista information page is here.
Finally, Microsoft provides its own list of third-party security vendors planning products for Vista. However, this link doesn't go to actual products; it's just a company Web-site directory.
Of course, as a disclaimer I should note that I don't mean to make light of what Microsoft's designers have done as far as building security into the operating system -- stuff like sandboxing to prevent corrupt apps from taking the system down. I won't belabor the discussion about the overly intrusive UACs, about which there's apparently little that can be done at this point.
However, I am making the point that the antivirus and anti-spyware ecosystem surrounding Vista is evolving rapidly and positively -- a development that's all the more significant given Microsoft's initial inclination to lock out third party vendors from providing products for Vista. Fortunately, the folks at Redmond came to their senses, doing a quick about-face in mid-October, opening up the Vista kernel so Symantec et al could update their products. Our list of fine freebies is the result.