The Joomla! Community Portal ™

In addition to the features described by Mark Dexter there is also a new "Purge Expired Cache" menu/feature in backend that will come especially handy to the large site administrators. It is a supplement to the "Clean Cache" that has been already there - with the difference that "Purge Expired Cache" will only clean expired items - cache files that are older than number of minutes set by Cache Time limit in Global Configuration. Items that are still current will remain intact - and that is a real advantage of this option - cleaning them will not affect Joomla!'s performance gains that come from frontend  caching.

I would advise all administrators of Joomla! powered sites to use new option from time to time as number of unused cache files remaining in cache folder can rise very high especially with some components that use large number of URL parameters (e.g. events components). This happens because cache files are created on the URL basis - each combination of parameters creates new URL and each of them creates new cache file file - combined with web spiders like Google that try to index each and all of them numbers can get very high. There is another problem associated with events components (or any components dealing with items valid only for limited period of time) -  if event's don't expire numbers will rise fast - but still serve it's purpose - but if they do, you will get high number of unused cache files for any of the expired events (core com_content takes care of this and related problems by cleaning it's cache on every save).

Also if you make a new menu that links to the component and delete old one this will effectively change Itemids for all URL-s created by the component. As Itemid is a part of the URL this will create a whole new set of cache files,  but old ones will remain indefinitely - until you either clean or purge the cache.