I found a thread using the search which talks all about it here: http://priuschat.com/forums/gen-iii-2010-prius-main-forum/67313-plasmacluster.html It is also in the manual.
That thread is basically a rant on smoking while pumping gas with a reference to yet another thread that doesn't say much and a page from the manual which doesn't say much. Here is a link from Sharp which has a little more description but makes the whole thing sound a lot like snake oil: SHARP Basically it is an air ionizer which creates ozone which may or may not be healthy but also imparts charge to dust allowing it to stick to surfaces rather than float around in the air.
