I know that we have information about Snort already on the site, but I thought that this article from Security Focus might prove useful. It is listed as part 1, so I will post part 2 when it comes out on the site. I have included an excerpt, but the full article is here . They bring up a major point about the cost - a plus, obviously.

Intrusion detection systems (IDS) are one of the fastest growing technologies within the security space. Unfortunately, many companies find it hard to justify acquiring IDS systems due to their perceived high cost of ownership (for example see Justifying the Expense of IDS by Kevin Timm and David Kinn). However, not all IDS systems are prohibitively expensive. This two-part article will provide a set of detailed directions to build an affordable intrusion detection architecture from hardware and freely available software. This discussion will avoid the classic "build or buy" debate and instead focus on building the system at a minimum cost.