Create a random 3D asteroid
Blender + Python
Blender's built-in Python scripting is perfect for this sort of thing.

Open a Python console and paste in the following:
import bpy
import random

def asteroid():
    bpy.ops.mesh.primitive_ico_sphere_add()
    bpy.ops.object.mode_set(mode='EDIT')
    bpy.ops.mesh.faces_shade_smooth()
    bpy.ops.mesh.select_mode(type='VERT')
    bpy.ops.mesh.select_random()
    bpy.ops.transform.resize(value=(random.uniform(1.1,1.4),random.uniform(1.1,1.4),random.uniform(1.1,1.4)))
    bpy.ops.mesh.select_all(action='SELECT')
    bpy.ops.mesh.subdivide(smoothness=1)
    bpy.ops.mesh.select_random()
    bpy.ops.transform.resize(value=(random.uniform(1.05,1.15),random.uniform(1.05,1.15),random.uniform(1.05,1.15)))
    bpy.ops.mesh.select_all(action='SELECT')
    bpy.ops.mesh.subdivide(smoothness=1)
    bpy.ops.mesh.select_random()
    bpy.ops.transform.resize(value=(random.uniform(0.92,1.05),random.uniform(0.92,1.05),random.uniform(0.92,1.05)))
    bpy.ops.mesh.select_all(action='SELECT')
    bpy.ops.mesh.subdivide(smoothness=1)
    stretch = random.uniform(0.9,1.5)
    bpy.ops.transform.resize(value=(stretch,1,1))
    bpy.ops.transform.rotate(value=(random.uniform(-1.57,1.57)), axis=(random.uniform(-1.57,1.57),random.uniform(-1.57,1.57),random.uniform(-1.57,1.57)))
    bpy.ops.object.mode_set(mode='OBJECT')

asteroid()

An asteroid will then be created at the position of your 3D cursor:

Want another asteroid? Move the 3D cursor somewhere else and then type asteroid() into the Python console again (or just ↑Enter to repeat the last line entered).
To make it look more like a piece of space rock and less like a blob of plastic, you need to apply a suitable material. The following procedural material works quite well. I used a diffuse colour of #635C5B, and #8D887F in the marble texture.

How it works:

Start by creating a simple icosahedron (bpy.ops.mesh.primitive_ico_sphere_add()).
Enter edit mode (bpy.ops.object.mode_set(mode='EDIT')) and apply smooth shading so that the individual facets blend into one another (bpy.ops.mesh.faces_shade_smooth()).
Select a random subset of the object's vertices (bpy.ops.mesh.select_random()).
Scale them randomly in the X, Y and Z directions (bpy.ops.transform.resize()).
Select the whole object (bpy.ops.mesh.select_all(action='SELECT')) and apply smooth subdivision to add new vertices by interpolating smoothly across the deformed object (bpy.ops.mesh.subdivide(smoothness=1)).
(Repeat steps 3–5 a couple more times.)
Stretch the object in the X direction to make it a bit less spherical (bpy.ops.transform.resize(value=(stretch,1,1)))
Apply a random rotation transform (bpy.ops.transform.rotate())
Finally, exit edit mode and leave the object selected (bpy.ops.object.mode_set(mode='OBJECT')).

More examples: