I want to mention a funky tool that’s often missed in Blender.
Negative Metaballs. Add>Metaball as usual. Exit EDIT mode and Add>Metaball. This time before you exit EDIT mode, hit the Negative button in the EDIT buttons window. Then leave edit mode.
If you move the negative Metaball around, you can see the effect it has on the positive metaball.
Be careful though as negative metaballs are not displayed in the same manner as positive metaballs, you will only see the Pivot point not a meta-mesh.
This is a little test you can try to see the amazing effects negative metaballs can produce.
Make 1 Metaball, make it big.
Place three spheres (UV) inside, make them emit particles, one 100, one 200, one 300 particles.
Parent three negative metaballs, one to each sphere, and use duplivert on each sphere.
Make the 100 dup metaball quite big, the 200 medium and the 300 small. Hit ALT A to run animation in 3D window.
(Thanks to S68 at elYsiun.com for the example method. Click here for his negative metaball thread @ elYsiun. )