Here are the links to each version if iPhone, ipod, and ipad firmware:

iPhone 2g:
1.00, 1.01, 1.02, 1.1.1, 1.1.2, 1.1.3, 1.1.4, 2.0, 2.0.1, 2.0.2, 2.1, 2.2, 2.2.1, 3.0, 3.0.1, 3.1, 3.1.2, 3.1.3

iPhone 3g:
2.0, 2.0.1, 2.0.2, 2.1, 2.2, 2.2.1, 3.0, 3.0.1, 3.1, 3.1.2, 3.1.3, 4.0, 4.0.1, 4.1, 4.2.1

iPhone 3gs:
3.0, 3.0.1, 3.1, 3.1.2, 3.1.3, 4.0, 4.0.1, 4.1, 4.2.1, 4.3.1, 4.3.2, 4.3.3, 5.0, 5.1, 6.1, 6.1.2

iPhone4 GSM:
4.0, 4.0.1, 4.1, 4.2.1, 4.3.1, 4.3.2, 4.3.3, 5.0, 5.1, 6.1, 6.1.2

iPhone4 CDMA:
4.2.6, 4.2.7, 4.2.8, 5.0, 5.1, 6.1, 6.1.2

iPhone 4S
5.0, 5.1, 6.1, 6.1.2

iPhone 5
6.1, 6.1.2

iPad:
3.2.1, 3.2.2, 4.2.1, 4.3.1, 4.3.2, 4.3.3, 5.0,
5.1

iPad 2:
4.3.1, 4.3.2, 4.3.3, 5.0 (GSM), 5.0 (CDMA), 5.1 (Wi-Fi), 5.1 (GSM), 5.1 (CDMA), 6.1, 6.1.2

iPad 3:6.1, 6.1.2

iPod:
1.1.4,1.1.5, 2.1 , 2.2, 2.2.1

iPod 2:
2.1.1 , 4.0, 4.1, 4.2.1

iPod 3:
4.0, 4.1, 4.2.1, 4.3.1, 4.3.2, 4.3.3, 5.0, 5.1

iPod 4:
4.1, 4.2.1, 4.3.1, 4.3.2, 4.3.3, 5.0, 5.1

Apple TV:
4.3.1, 4.4

Custom Firmwares (for pwnage) these are not official:
Several custom firmwares can be found here

Bootloader Images:

Zip with 3.9 and 4.6 in it here or here.

To install these firmwares, download the files in the browser then use iTunes to restore. First you will need to put your phone into DFU mode. Next, you will use the “shift restore” (PC) or “option restore” (OSX). This will bring up a file selection box in iTunes that allows you to find the file you downloaded to program. Remember, if you are downgrading your firmware then you will get an error at the end of the process such as 1011, 1013, 1015 or such. This is normal. Downgrading is moving from a higher firmware version to a lower version such as 1.1.4 to 1.1.1. Once you get the 101x error, you will need to boot your phone out of recovery using either independence (OSX), iBricker (PC) or jailbreak 1.1.2 (PC).