Hi, in my blog I make use of hierarchies for categories, for instance Sport -> Football -> Premiership, and when I write a post, I like it to be in all 3 categories. So, to save time clicking three boxes, I have created a plugin that means you can click on a child category and all it's parents are check, for instance in the above example, checking Premiership, would also check Football and Sport.
You can try it at: http://www.pete-b.co.uk/downloads/#WP
It's only in version 0.1 currently, but I would appreciate feedback.
PS: I'm not sure if this was already done by someone, if it has been sorry :)