I found this article over on the Datamation Blog that lists 50 great opensource and freeware security solutions.  It’s an impressive list of tools that fill almost every security need you might have.  One of the cool things about the article is that they tell you what the software is a possible replacement for.  The tools on the list are at varying levels of maturity so be sure to do more research before replacing a key piece of your security infrastructure with them.