iPhone OS 4 was announced on Thursday, and its got a lot of great new features:

Multitasking

• You can now multitask between apps. The UI is very well designed and is accessed by double tapping the home button.
• Audio apps like Pandora can now continue to play in the background.
• VOIP apps such as Skype can receive calls when you’re doing something else, and you can switch to another app without dropping a call.
• Apps can now receive notifications when your location changed which are triggered when you change cell towers.
• Apps can now display local notifications which are like push notifications but are generated locally by the app.

Folders

• You can now create folders on the home screen which allow you to group apps however you like. This is a great feature which has been needed for a long time!
• You can put folders in the dock.

Wallpaper

• You can now change the wallpaper for the home page and the lock screen.

Mail

• Unified inbox.  Messages from multiple accounts now appear in one inbox.
• Organize messages by thread.
• Open attachments.

iAd

• Developers can easily add ads to their apps and earn 60% of the revenue.
• Users access interactive ads within apps and can return to the app where they left off.

Other Stuff 

• iBooks is now available on iPhone