Wikipedia has a comprehensive list of camouflage patterns, and the nations that use them. I was surprised by the number of nations using good old USA Woodland Camo, among them being our friends the Chinese.
Kamouflage.Net has images of 297 camo patterns from 92 countries.
Another site to visit is Camopedia.
One should take the time and read a rant over at MILSPECMONKEY about camo patterns and their drawbacks. Especially helpful in explaining just what the author is talking about are the numerous photos that illustrate the shortcomings of many of the new camo patterns.
The photos highlight that the most effective camo is the one designed specifically for the environment. Unfortunately, most military units have to do with off the shelf gear and clothing. The cost of resupplying each soldier with terrain specific (urban, grassland, woodland, forested, etc.) clothing and gear would be prohibitive, not to mention being a logistical nightmare. Although, I understand some special operations groups are provided with such.
Therefore, most fighting men are equipped with what amounts to general purpose gear designed to work somewhat well in most environments, but not optimized for each environment. As an example, here in California I can be in San Francisco (a decidedly very urban environment) and in twenty minutes be in some of the most dense redwood forests to be found, and in another thirty minutes be in rolling grasslands that would remind someone of western Kansas and eastern Colorado. Between the two? Hills densely wooded with Oak and Manzanita.
A couple hours east, it is fir, pine, granite and snow.
What is a quartermaster to do? Urban camo? Woodland? MARPAT? Multicam?
More than likely they'll issue UCP, universal camo pattern
Many people are concerned about the very real possibility of martial law being declared within the borders of the United States. Since resistance will initially be local, it would seem to be advantageous to optimize your gear to best enable you to blend into your immediate surroundings.
Don't forget, that dressing like local everyday people can be an effective camouflage. This technique has been proven effective on battlefields in our own war for independence, as well as today in Iraq and Afghanistan.
Places like ReelFoot and Tactical Camo sell stencils and paint to allow you to customize your hardware, but what about just doing it yourself?
Watch this video:
As one can see from this video, one doesn't need to buy expensive stencils. You don't even need an airbrush setup. You can use spray paint and home made stencils.
For weapons, check out DuraCoat for the ultimate finish.
But for everything else and the down and dirty, get it done now expedient method (yes, in a pinch you can spray paint clothing), check out Krylon's Outdoor Spaces and Krylon's line of Camouflage Paint.
Krylon offers free stencils on its site for download and also gives tips here.
You can make your own stencils using acetate paper (found on line and in art shops) and your printer. Locate camo clip art or images of the stencils you wish to use (CLICK HERE) and print them out. You may need to resize the images you find. An excellent free program for manipulating images is GIMP. Read about GIMP here.
The ink may take as long as 10 minutes to dry on the acetate, so be patient.
Cut out the stencils and spray away. I recommend using a scrap piece of plywood or some cardboard to practice on to get the hang of laying down a pattern.
In a pinch, one can also grab local grasses, leaves and branches to use as stencils.
Camouflage Effectiveness gives the reader a basic rundown on how modern hunting camouflage works.
Create your own custom digital camo at this site:
Custom Random Digital Camouflage Generator