WetPaint (http://wetpaint.com ) might be the easiest to understand wiki I've run into. While it's still in beta testing, you can go to the site, try out some of the features (or sit through their own Flash-demo) or visit sites using WetPaint.
There's something pleasingly 'contained' feeling about this site -- no small accomplishment because wikis often give you a sense of being in the un-governable Wild West. That rabble-y feeling is - regulary - exactly the reason most people either 'don't get' wikis or, worse, don't trust them for 'real work.'
Wet Paint may change that.
