This individual attempts to address the free rider problem of military defense in anarchy.
[youtube]http://www.youtube.com/watch?v=K1ON-L9UR-o&feature=feedu[/youtube]
I don't really see a free rider problem. I expect the places under the most threat of some violent act would be under some specialized insurance, places like airports, sea ports, borders to dangerous areas, nuclear facilities, etc.
The main difference I see is that I don't see a need for maintaining an army. The market allows for firms to quickly change production to meet demands. If an area under ground invasion, or under immediate threat, the local law enforcement agencies would respond. I expect the property owners to have some form of law insurance, so to maintain the safety the insurance companies would buy the services of law enforcement from other areas not under immediate threat (price signals would allocate the soldiers/police to where needed).
The same with military equipment. Most military technology can be produced easily by most firms, factories that make non-military goods could shift their production to equivalent military goods. War is costly, and instead of a state, the market would ensure the capital goods and individuals would allocate most efficiently to meet individuals demand.
There's also the fact that I imagine the sky will be owned by the homesteading principle, airplane industries will own the legal rights to airspace. Given the threat to their planes, planes contracted from other firms to fly in that airspace, and the individuals on the planes (who will most likely be assures some level of safety), an air firm would probably want some sort of safety precaution. There's also the fact that planes create pollution and noise pollution, so to prevent any legal threats from land owners against the sky owners, trespassers would have to be dealt with. I imagine some companies that respond to foreign trespassers by shooting them down. But given the benefits of other air firms using this airspace to travel, there is an incentive to be very sure before shooting anything down.
So basically, I don't see a free rider problem. Not all individuals in a society are under the same threats. If troops invade my half of Pennsylvania, individuals on the other half of PA would not be in as much harm, so I don't see how they are really a free rider. I assume my direct neighbors already pay for some form of law insurance, and I imagine their law insurance would react similar to mine. Instead of a strong static blanket of protection, I see military defense as more of a smart net, responding to the areas of intrusion. The areas under the most threat of invasion would have a greater demand for military defense, so capital would naturally allocate to where it's most profitable.
Sound about right?