I'm using the 'Nivo Slider for Wordpress' and the 'JJ NextGen JQuery Carousel' plugins on http://www.whiteecho.com
Each plugin works on its own but when you activate the Nivo Slider for Wordpress, the Carousel stops working.
The Nivo slider loads jquery from its own source as opposed to using wp_enqueue_scripts. This will break any other jquery based plugins that have not yet initialised as it overwrites the previous jquery plugin files.
The author has apparently produced a patch to fix it