Apple has released the Gold Master, or GM seed of iOS 9. Assuming there are no show-stopping bugs, this will be the final pre-release version of iOS 9 before its public release on September 16. Developers can grab the release from Apple's iOS Developer Resource Center.
Apple has also released the GM seed of of watchOS 2. It can be downloaded on an Apple Watch that has the most recent developer beta build installed, using the Apple Watch companion app on an iPhone running iOS 9.
When it arrives next week, iOS 9 is meant both to boost performance, and improve the overall user experience. Highlights of iOS 9 include a more intelligent Siri, search improvements, an iCloud Drive-powered Notes app, and smaller download sizes.