This clip from Animal Planet has a couple of good examples of parasites in action*:
And this one from the BBC’s Planet Earth shows the Cordyceps parasite take over an ant:
This clips from NewScientist shows the jewel wasp turning a cockroach into its slave/zombie/mobile dining centre – scary!
*Sadly, the best clips have been removed from GoogleVideo. You can buy the 3-part series from Discovery, for the bargain price of $449.85. I still don’t understand why some producers have to charge such a ridiculous fee for their programmes – surely that restricts their use in education? Hats off to all the documentary makers who post them up for free after a while (such as The 11th Hour and SuperSizeMe).