People make spark plugs out of the craziest elements, like Polonium. Platinum is used because it doesn't corrode under harsh conditions.
I remember hearing, probably on NPR's Car Talk show, that platinum spark plugs last so long that by the time you need to change them, they may have fused themselves to the aluminum block they are screwed into, making it impossible to remove them without stripping the engine block. They recommended just unscrewing and rescrewing them every couple of years. Ah, such modern problems.
Here's an interesting article about different elements in spark plugs.
Source: eBay seller accurateimage
Contributor: Theodore Gray
Acquired: 31 July, 2002
Text Updated: 11 August, 2007
Sample Group: Spark Plugs