Now that you have separated the boards and driven out the nails you have nice pile of wood. If the boards aren't as straight as you might like re-cut ...
Recycling wood is a great way to get free wood, keep useful things out of landfills, and save a few trees. In areas with few resources, other than resourceful people, recycled wood is used for cooking, heating, art, toys, tools, furniture, and construction.
The first step is to find some wood. Most industrial areas will have stacks of pallets that they are willing to give you, wooden objects often are left on the street on garbage day, dumpster dive, see what washes up on the beach, or scavenge from that project you abandoned.
Step 2: Tools
In a pinch all you need is a hammer and willpower. However, a crowbar is very useful, safety glasses will keep flying nails out of your eyes, and ear protection will soften the pounding sounds. If you want to get really fancy I also recommend a cat's paw and pliers for pulling out especially difficult nails, and a saw to cut around the impossible ones (none of these additional items are pictured because they are usually not necessary).
Step 3: Lift one side
Place the crowbar between the planks of wood in line with the nails holding it together, and lift. This should pull the nails partway out. I do not recommend trying to pull the nails all the way out with out first loosening the other side, pulling too hard may result in cracked wood.
Step 4: Lift the other side.
Move the crowbar to the other side, and pull and loosen it. Switch back and forth until one side is free. If you switch more than three times you are not exerting enough force.
Step 5: Lever out the other side
Once one side is free you can use it to lever out nails on the other side by lifting the whole plank.
Step 6: Pound out the nails
Flip the louse board over and pound the sharp end of the nails with the hammer. This will force the nails most of the way out of the wood.