Glenn Reynolds outlines the legalities of asteroid mining.
Asteroids are certainly available, and they're valuable. More than 750,000 asteroids measure at least 1 kilometer across, and millions of smaller objects are scattered throughout the solar system, mostly in the asteroid belt between Mars and Jupiter. Even a comparatively small asteroid is potentially quite valuable, both on Earth and in space.
A 79-foot-wide M-type (metallic) asteroid could hold 33,000 tons of extractable metals, including $50 million in platinum alone. A 23-foot-diameter C-type (carbonaceous) asteroid can hold 24,000 gallons of water, useful for generating fuel and oxygen. Even 1 gallon of water, at 8.33 pounds per, can cost tens of thousands of dollars to launch into Earth orbit. Prices will probably come down now that SpaceX and other private launch companies are in the game. But the numbers would need to improve a lot for water launched from Earth to compete with water that's already floating in space.
Larger asteroids could be worth as much as the GDP of a superpower. Asteroid 1986 DA is a metallic asteroid made up of iron, nickel, gold, and platinum. Estimates of its value range between $6 and $7 trillion. Something that size won't be retrieved anytime soon, but the figure gives some idea of just how much wealth is out there.
Ok, so that's just a quote about how valuable asteroids are. If you're interested in the existing legal issues go read the article, but the bottom line is that anything you can move is up for grabs.