Apple has finally released iOS 4, which is available as a software update in iTunes. The update is free for iPod Touch and iPhone users, but the compatible models are: iPhone 3G (no multitasking), iPhone 3GS, iPhone 4 and 2nd / 3rd generation iPod Touches.

iOS 4 is one of the most important releases for Apple this year, and it comes with 100 new features including multitasking, folders, enhanced Mail.app, wallpapers, iBooks, enhanced Photos.app, better Enterprise support and digital zoom. iOS 4 also includes support for Apple’s mobile advertising platform iAd, which is set to launch on July 1st.

Back in April Apple announced that iOS 4 for iPad would come this fall, but we haven’t heard anything else about it since then.

UPDATE: We’ll insert the direct link to iOS 4 as soon as we manage to get it.

UPDATE #2 Here are the direct links: (and yes, it seems like the final build is the same seeded at the WWDC - 8A293)

iPhone 3GS Link

iPhone 3G Link

iPhone 4 Link

iPod Touch 2G Link

iPod Touch 3G Link

UPDATE #3 Check out the full Readme file below:

This update contains over 100 new features, including the following:

- Multitasking support for third-party apps*

- Multitasking user interface to quickly move between apps

- Support for audio apps to play in the background

- VoIP apps can receive and maintain calls in the

background or when device is asleep

- Apps can monitor location and take action while running in the background

- Alerts and messages can be pushed to apps using

push and local notifications

- Apps can complete tasks in the background

- Folders to better organize and access apps

- Home screen Wallpaper*

- Mail improvements

- Unified inbox to view emails from all accounts in one place

- Fast inbox switching to quickly switch between different email accounts

- Threaded messages to view multiple emails from the same conversation

- Attachments can be opened with compatible third- party apps

- Search results can now be filed or deleted

- Option to select size of photo attachments

- Messages in the Outbox can be edited or deleted

- Support for iBooks and iBookstore (available from the App Store)

- Photo and Camera improvements

- 5x digital zoom when taking a photo**

- Tap to focus during video recording**

- Ability to sync Faces from iPhoto

- Geo-tagged photos appear on a map in Photos

- Ability to create and edit playlists on device

- Calendar invitations can be sent and accepted wirelessly with supported CalDAV servers

- Support for MobileMe calendar sharing

- Suggestions and recent searches appear during a web search

- Searchable SMS/MMS messages**

- Spotlight search can be continued on web and Wikipedia

- Enhanced location privacy

- New Location Services icon in the status bar

- Indication of which apps have requested your location in the last 24 hours

- Location Services can be toggled on or off for individual apps

- Automatic spellcheck

- Support for Bluetooth keyboards*

- iPod out to navigate music, podcasts and audiobooks through an iPod interface with compatible cars

- Support for iTunes gifting of apps

- Wireless notes syncing with IMAP-based mail accounts

- Persistent WiFi connection to receive push notifications*

- New setting for turning on/off cellular data only**

- Option to display the character count while composing new SMS/MMS**

- Visual Voicemail messages can be kept locally even if

they have been deleted from the server**

- Control to lock portrait orientation*

- Audio playback controls for iPod and third-party audio apps*

- New languages, dictionaries and keyboards

- Accessibility enhancements*

- Bluetooth improvements

- Better data protection using the device passcode as an encryption key* (Requires full restore.)

- Support for third-party Mobile Device Management solutions

- Enables wireless distribution of enterprise applications

- Exchange Server 2010 compatibility

- Support for multiple Exchange ActiveSync accounts

- Support for Juniper Junos Pulse and Cisco AnyConnect SSL VPN apps (available from the App Store)

- More than 1,500 new developer APIs

- Bug fixes

Products compatible with this software update:

- iPhone 3G

- iPhone 3GS

- iPhone 4

- iPod touch 2nd generation

- iPod touch 3rd generation (late 2009 models with 32GB

or 64GB)

* Requires iPhone 3GS, iPhone 4, and iPod touch 3rd generation.

** Requires iPhone 3G, iPhone 3GS, and iPhone 4. SMS/MMS messaging and Visual Voicemail require support from your wireless carrier.

For feature descriptions and complete instructions, see the user guides for iPhone and iPod touch at:

http://support.apple.com/manuals/iphone

http://support.apple.com/manuals/ipodtouch

For more information about iPhone and iPod touch, go to:

http://www.apple.com/iphone

http://www.apple.com/ipodtouch

To troubleshoot your iPhone or iPod touch, or to view additional support information go to:

http://www.apple.com/support/iphone

http://www.apple.com/support/ipodtouch

For information on the security content of this update, please visit this website:

http://support.apple.com/kb/HT1222

UPDATE # 4 Yes, Game Center has been pulled from the final release and won’t be back until this fall, it seems. It was just a developer preview.

UPDATE # 5 For those wondering whether they should upgrade from the GM seed: even though the two builds are basically the same thing, the final one has a new SHA1 checksum. That’s the change.

UPDATE #6 Many users are reporting activation issues and iTunes errors. Just keep trying, but remember that it’s going to take a while anyway.

UPDATE # 7 Nik Fletcher has collected  8 subtle changes of iOS 4.