Apple's mobile operating system, iOS, released its biggest update in quite some time today. Now available for download via iTunes, iOS 5 comes with more than 200 new features for iPhone, iPad and iPod Touch devices.

Among the most significant of the enhancements is iCloud, Apple's new wireless content syncing feature, which Steve Jobs unveiled along with the rest of iOS 5 at the World Wide Developer's Conference in June. The new service allows users to sync contacts, photos, calendars, music, movies and other content across devices and back everything up safely online. The upgrade also eliminates the need to plug Apple's mobile devices into a computer to get software updates and sync content.

The new operating system also features revamped user notifications, deep Twitter integration, cross-device to-do manager, an SMS-style messaging system and a digital newsstand for publishers. iTunes in the Cloud allows users to back up their entire music library on Apple's servers without having to upload thousands of files.

Enhancements have also been made to the camera, which this version of the OS makes much easier to access and use. That's something that's sure to excite Instagram and Hipstamatic addicts, along with anybody who frequently uses their phone to take pictures but is sick of missing shots. Once the picture is snapped, users can now edit them on the phone using the cropping, resizing and red eye removal tools found in the Photos app.

The Safari Web browser is also getting a few badly-needed upgrades, including tabbed browsing and an Instapaper-style "read it later" function.

There are so many substantial upgrades to the new OS that some of the smaller details get lost in all the coverage. iOS 5 brings a number improvements to Mail, Calendar and Game Center, as well a redesigned Music app for iPad, weather forecasts and a split keyboard for iPad users. Other upgrades to the keyboard in iOS 5 include the option to create shortcuts for commonly used words and a personalized dictionary for autocorrection.

The iOS 5 update comes just a week before the new iPhone 4S starts shipping. That hardware upgrade will bring a faster CPU, better camera and a voice-activated "virtual personal assistant" but there's enough packed into iOS 5 to keep users busy in the meantime.

iOS 5 is available on iPhones as far back as the 3GS, so users who are still hanging on to one of the first two iterations of the device will need to upgrade to a newer model to enjoy the latest version of the operating system. Apple has made that relatively easy (depending on your carrier contract) by dropping the price of the iPhone 4 to $99 and making the 3GS available for free.

Both models of the iPad are also eligible for the upgrade, as are the third and fourth generation iPod Touches.

What features are you most excited about in iOS 5? Anything you think it's missing? Let us know your thoughts in the comments.