I really like Bryce's metaballs.. it does take some practice to get something right, but it's a lot of fun too. Adding color to one ball and a different color to another makes some nice blending effects.

You might want to try out CB Modeler Pro too for similar model effects. It's not metaball, yet used with metaballs, one can model some very nice stuff. The models in CB Modeler Pro tend to be hollow. Here's the link to it, and it is free. http://www.cbmodelpro.com/