Back in September I mentioned that GFI LanGuard was available for free for small companies or home use where you only needed to scan 5 PCs.
One other option that has come up since then is the new much easier to use web-based Nessus 4.2.
Nessus has always been free for home users, but now I feel that it's easy enough for most home users to set up. It comes in a windows version, and there is only the server end to set up now, everything else is done through a browser.
Unfortunately the Pro version of Nessus is a little pricey for the average small business at $1200 per year, but you can hire a pro, like me, to come in and scan your network on a regular basis with this tool for probably a fair bit less than that. (pro licenses are not tied to a physical network, but limited to one machine... so if that machine is a laptop, a pro feed license can go wherever the security contractor takes it.)
Rapid 7 has also recently released NeXpose Community Edition, which I have yet to try out, but is free to use for a network of up to 32 PCs, and there is the open source OpenVAS, which was spun off from Nessus back at version 2, when Nessus was still an open source project. These 2 options I suspect would be more difficult to get up and running than the first two, as they are really aimed at folks with a high level of tech knowledge. NeXpose comes in several other versions for varying levels of additional features, and larger networks, but it is more expensive than the Nessus Pro feed, so very much out of the reach of the average small business or home user, but the Community edition is supposed to be very good, and I'll be playing around with it in the next few weeks and I will let you all know what I think.
No matter which you choose, scanning your network, especially for business networks, is an important part of keeping your network secure. If you don't scan it to find the holes in your security, someone else will, and they probably won't point out the holes to you, they'll probably just use those holes in ways you don't want them to.
One other option, from the folks at Rapid 7 is the free online scan. You can scan 2 IP addresses for free from the internet at http://www.rapid7.com/freescan.jsp This should give you an idea of how exposed your servers that are attached to the internet are. This will only scan public Internet IP addresses. It is probably best to get a local scanner set up or hire a pro to come in and scan the private address space as well, especially if you use wireless.