Next version of iOS with overhauled notifications, lock screen, Messages, Apple Music, and much more.
iOS 5 Beta 6 Seeded to Developers [Updated]
The over-the-air version of iOS 5 beta 6 for the iPhone checks in at 144 MB. (Thanks, Parker!)
In addition to iOS 5 beta 6, Apple has also released a new beta version of iTunes 10.5 and a new preview version of Xcode 4.2 to support the release.
Apple is expected to release iOS 5 for existing hardware around the time of the iPhone 5 launch, with rumors for the debut now coalescing around early October.
Update: Apple has also pushed out iWork for iOS beta 2 and a new "beta 5" of the Apple TV software.
Update 2: As noted by It's All Tech, Apple on the Mac side has pushed out a new developer seed of OS X 10.7.2 (Build 11C40), iCloud for OS X Lion beta 8, and a new build of Safari 5.1.1, all to be used for testing out iCloud.
Update 3: iLounge notes that Apple appears to have disabled Nike+ syncing through iTunes for iOS 5 users with the new beta, forcing them to use the wireless syncing option that debuted in iOS 4.1.