Anyone know if deleting all cookies clears all cache or just cookies? Want to mention that that method also might only work if the cookie is visible to the browser (e.g. can see it with browser dev tools), so you might have to start on the right page before deleting all cookies (e.g. site home page) and not delete cookies at start of test before even navigating anywhere (e.g. blank page). That is definitely the case with Selenium RC, not sure if that was ever changed for WebDriver.
Also, if needing to delete other types of cache like HTML5 local or session storage, you can do it via javascript calls to the HTML5 local/sessionStorage object API to clear them out. (And refresh page afterwards, etc. to see the change reflected).