How to model simple URDF objects?

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Try grabbing the latest Gazebo (version 1.2). The default install will allow you drag-and-drop a hammer into simulation.

Here is the SDF we use for the hammer:

<?xml version="1.0"?>
<gazebo version="1.2">
  <model name="hammer">
    <static>false</static>
    <link name="link">
      <collision name="collision">
        <geometry>
          <mesh>
            <uri>model://hammer/meshes/hammer.dae</uri>
          </mesh>
        </geometry>
      </collision>

      <visual name="visual">
        <geometry>
          <mesh>
            <uri>model://hammer/meshes/hammer.dae</uri>
          </mesh>
        </geometry>
      </visual>
    </link>
  </model>
</gazebo>

An improvement can be made by using simple shapes for the collision object rather than the mesh itself.