From time to time you may need a version of IOS which is no longer the current firmware for your device. Below are links to the Apple download servers for all versions of IOS for the iPhone. Downloading these original IOS versions from Apple servers is considered the only legitimate source for official stock firmware.

IMPORTANT NOTE - Downloading one of these firmware files does not mean you can simply install it on your device. For iPhone3GS and newer, you also need Saved SHSH Blobs which are specific to your device and the version of IOS you are going to install. If you didn't take the time to save these blobs when Apple released the firmware there is nothing you can now do to downgrade back to an older version. Sorry.

iPhone4 GSM
4.3.5 http://appldnld.apple.com/iPhone4/04...1_Restore.ipsw
4.3.4 http://appldnld.apple.com/iPhone4/04...2_Restore.ipsw
4.3.3 http://appldnld.apple.com/iPhone4/04...2_Restore.ipsw
4.3.2 http://appldnld.apple.com/iPhone4/04...7_Restore.ipsw
4.3.1 http://appldnld.apple.com/iPhone4/04...4_Restore.ipsw
4.3 http://appldnld.apple.com/iPhone4/04...0_Restore.ipsw
4.2.1 http://appldnld.apple.com/iPhone4/06...8_Restore.ipsw
4.1 http://appldnld.apple.com/iPhone4/06...7_Restore.ipsw
4.0.2 http://appldnld.apple.com/iPhone4/06...0_Restore.ipsw
4.0.1 http://appldnld.apple.com/iPhone4/06...6_Restore.ipsw
4.0 http://appldnld.apple.com.edgesuite....3_Restore.ipsw


iPhone4 CDMA
4.2.10 http://appldnld.apple.com/iPhone4/04...0_Restore.ipsw
4.2.9 http://appldnld.apple.com/iPhone4/04...1_Restore.ipsw
4.2.8 http://appldnld.apple.com/iPhone4/04...1_Restore.ipsw
4.2.7 http://appldnld.apple.com/iPhone4/04...3_Restore.ipsw
4.2.6 http://appldnld.apple.com/iPhone4/04...0_Restore.ipsw


iPhone3GS
4.3.5 http://appldnld.apple.com/iPhone4/04...1_Restore.ipsw
4.3.4 http://appldnld.apple.com/iPhone4/04...2_Restore.ipsw
4.3.3 http://appldnld.apple.com/iPhone4/04...2_Restore.ipsw
4.3.2 http://appldnld.apple.com/iPhone4/04...7_Restore.ipsw
4.3.1 http://appldnld.apple.com/iPhone4/04...4_Restore.ipsw
4.3 http://appldnld.apple.com/iPhone4/04...0_Restore.ipsw
4.2.1 http://appldnld.apple.com/iPhone4/06...a_Restore.ipsw
4.1 http://appldnld.apple.com/iPhone4/06...7_Restore.ipsw
4.0.2 http://appldnld.apple.com/iPhone4/06...0_Restore.ipsw
4.0.1 http://appldnld.apple.com/iPhone4/06...6_Restore.ipsw
4.0 http://appldnld.apple.com.edgesuite....3_Restore.ipsw
3.1.3 http://appldnld.apple.com.edgesuite....8_Restore.ipsw
3.1.2 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
3.1 http://appldnld.apple.com.edgesuite....4_Restore.ipsw
3.0.1 http://appldnld.apple.com.edgesuite....0_Restore.ipsw
3.0 http://appldnld.apple.com.edgesuite....1_Restore.ipsw


iPhone3G
4.2.1 http://appldnld.apple.com/iPhone4/06...8_Restore.ipsw
4.1 http://appldnld.apple.com/iPhone4/06...7_Restore.ipsw
4.0.2 http://appldnld.apple.com/iPhone4/06...0_Restore.ipsw
4.0.1 http://appldnld.apple.com/iPhone4/06...6_Restore.ipsw
4.0 http://appldnld.apple.com.edgesuite....3_Restore.ipsw
3.1.3 http://appldnld.apple.com.edgesuite....8_Restore.ipsw
3.1.2 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
3.1 http://appldnld.apple.com.edgesuite....4_Restore.ipsw
3.0.1 http://appldnld.apple.com.edgesuite....0_Restore.ipsw
3.0 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
2.2.1 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
2.2 http://appldnld.apple.com.edgesuite....7_Restore.ipsw
2.1 http://appldnld.apple.com.edgesuite....6_Restore.ipsw
2.0.2 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
2.0.1 http://appldnld.apple.com.edgesuite....8_Restore.ipsw
2.0 http://appldnld.apple.com.edgesuite....7_Restore.ipsw


iPhone (Original)
3.1.3 http://appldnld.apple.com.edgesuite....8_Restore.ipsw
3.1.2 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
3.1 http://appldnld.apple.com.edgesuite....4_Restore.ipsw
3.0.1 http://appldnld.apple.com.edgesuite....0_Restore.ipsw
3.0 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
2.2.1 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
2.2 http://appldnld.apple.com.edgesuite....7_Restore.ipsw
2.1 http://appldnld.apple.com.edgesuite....6_Restore.ipsw
2.0.2 http://appldnld.apple.com.edgesuite....1_Restore.ipsw
2.0.1 http://appldnld.apple.com.edgesuite....8_Restore.ipsw
2.0 http://appldnld.apple.com.edgesuite....7_Restore.ipsw
1.1.4 http://appldnld.apple.com.edgesuite....2_Restore.ipsw
1.1.3 http://appldnld.apple.com.edgesuite....3_Restore.ipsw
1.1.2 http://appldnld.apple.com.edgesuite....b_Restore.ipsw
1.1.1 http://appldnld.apple.com.edgesuite....a_Restore.ipsw
1.0.2 http://appldnld.apple.com.edgesuite....8_Restore.ipsw
1.0.1 http://appldnld.apple.com.edgesuite....5_Restore.ipsw
1.0.0 http://appldnld.apple.com.edgesuite....a_Restore.ipsw


Happy firmware upgrading / downgrading / jailbreaking! :D