Apple made iOS 4 available for free download today for owners of newer iPhone and iPod touch models. The revised operating system, first announced in April 2010 as iPhone OS 4, and renamed to iOS 4 earlier this month, adds a host of features, including specific kinds of multitasking, external keyboard support, a unified Mail inbox, and folders for organizing apps, among many other smaller changes; iBooks 1.1 is also now available as a separate free download from the App Store. An iPhone 3GS or third-generation iPod touch is required for multitasking and using external keyboards.

We've been covering iOS 4 as features have been announced. You can read "Apple Previews Major New Features in iPhone OS 4," (8 April 2010) for a rundown of what was known before release. TiPb has an extensive walkthrough of all the new features if you want to know a little about every single thing that's changed.

The most notable single improvement is multitasking, in which several kinds of activities can continue to function in the background while you're using other apps. These activities include continuing a VoIP phone call, playing music, finishing a download, and updating location for navigation apps.

App developers must update their software to take advantage of multitasking, and Apple started accepting revised iOS 4 apps on 10 June 2010. Companies will likely start promoting specific improved features as their updates are made available in the App Store.

The iOS 4 download (378 MB for iPhone, 315 MB for iPod touch) is available via iTunes; there's no fee for iPod touch upgrades following an accounting change by Apple last year. Connect your iPhone 3G or 3GS, or iPod touch second- or third-generation model, to the computer with which you sync iTunes data. (The original iPhone and first-generation iPod touch cannot be upgraded to iOS 4.) Then select the device in the iTunes sidebar, and click Check for Updates in the Summary tab. Beware that it may take quite some time to download and install the multi-hundred-megabyte update.