Compatible with iPhone 3GS, iPhone 4, iPhone 4S, iPod Touch 3G, iPod Touch 4G, the first-gen iPad, iPad 2, and Apple TV (4.4) – iOS 5 brings over 200 new features and improvements including the Notification Center, Newsstand, Reminders App, deep Twitter Integration, Optimized Safari Web Browsing, Enhanced Camera App, iMessage, an updated Game Center, iCloud Sync, Multi-tasking Gestures, as well as Airplay Mirroring for iPad. iOS 5 also has the ability to update itself over-the-air (OTA), like whenever new minor updates are available.
iOS 5 can be downloaded through iTunes; please note though that you will need the latest version of iTunes to date (v10.5) before you can download the latest firmware for your device. You can get iTunes 10.5 from here.
For those who want to download iOS 5 for later upgrade, you can grab one directly from Apple servers through the links below.