Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Today I took a moment to consider the myriad of really cool work happening (mainly) in the open source tool community. When I started counting, I found about seven projects that are likely to help you defend your enterprise.
Most of these require some commitment of brainpower and willingness to learn, but I am nevertheless very pleased to see this much innovation on the defensive side. Collectively these projects do not "solve" any problems (nor should they), but I am certain they can help address one or more problems you may encounter -- especially regarding visibility. In other words, these are the sorts of tools (with one or two exceptions) that will help you detect and respond to intruders.