For the first time for a major iOS release, Apple is today making a beta version available to the public. Interested users can sign up to download the beta here (once the page goes up at around 2PM ET) and then load it onto an iPhone or iPad with iTunes. Now, of course, to the caveats: it's beta software and so the vast majority of people shouldn't install it on their primary phones. Though likely stable enough, it will still be replete with bugs (and sometimes reduced battery life) that's always associated with pre-release software. Apple says that it will be issuing "periodic" updates to the beta. Apple's last public beta for iOS came earlier this year, with iOS 8.3.
Update: Read the Apple iOS 9 review.
The point, of course, is for Apple to get actual feedback from users on the new software, and there's a built-in Feedback Assistant app to help with that. Apple has posted an FAQ with more details about the program and how it works (including how to roll back to the backup of iOS 8 that you obviously created before you installed the beta).
iOS 9 has a few major new features, the most interesting of which is a set of "proactive" features that allows Siri to automate tasks like adding calendar entries or suggesting playlists. It also features a "low power mode" for extending battery life, transit directions in Apple Maps, and a new News app. The iPad gets a new Split View that allows you to use two apps at once. The full version usually gets released in the fall alongside new iPhones, and we don't see any reason why this year would be any different.
WWDC 2015 iOS 9 preview