Below you can find the direct links to the iPhone Firmware Files for every released firmware version. Please note that if you use Safari you must disable the auto unzip feature. It may be easier to just use Firefox!
1.0.0: iPhone1,1_1.0_1A543a_Restore.ipsw
1.0.1: iPhone1,1_1.0.1_1C25_Restore.ipsw
1.0.2: iPhone1,1_1.0.2_1C28_Restore.ipsw
1.1.1: iPhone1,1_1.1.1_3A109a_Restore.ipsw
1.1.2: iPhone1,1_1.1.2_3B48b_Restore.ipsw
1.1.3: iPhone1,1_1.1.3_4A93_Restore.ipsw
1.1.4: iPhone1,1_1.1.4_4A102_Restore.ipsw
2.0.0 (2G): iPhone1,1_2.0_5A347_Restore.ipsw
2.0.0 (3G): iPhone1,2_2.0_5A347_Restore.ipsw
2.0.1 (2G): iPhone1,1_2.0.1_5B108_Restore.ipsw
2.0.1 (3G): iPhone1,2_2.0.1_5B108_Restore.ipsw
2.0.2 (2G): iPhone1,1_2.0.2_5C1_Restore.ipsw
2.0.2 (3G): iPhone1,2_2.0.2_5C1_Restore.ipsw
2.1.0 (2G): iPhone1,1_2.1_5F136_Restore.ipsw
2.1.0 (3G): iPhone1,2_2.1_5F136_Restore.ipsw
2.2.0 (2G): iPhone1,1_2.2_5G77_Restore.ipsw
2.2.0 (3G): iPhone1,2_2.2_5G77_Restore.ipsw
2.2.1 (2G): iPhone1,1_2.2.1_5H1_Restore.ipsw
2.2.1 (3G): iPhone1,2_2.2.1_5H11_Restore.ipsw
3.0.0 (2G): iPhone1,1_3.0_7A341_Restore.ipsw
3.0.0 (3G): iPhone1,2_3.0_7A341_Restore.ipsw
3.0.0 (3GS): iPhone2,1_3.0_7A341_Restore.ipsw
3.0.1 (2G): iPhone1,1_3.0.1_7A400_Restore.ipsw
3.0.1 (3G): iPhone1,2_3.0.1_7A400_Restore.ipsw
3.0.1 (3GS): iPhone2,1_3.0.1_7A400_Restore.ipsw
3.1.0 (2G): iPhone1,1_3.1_7C144_Restore.ipsw
3.1.0 (3G): iPhone1,2_3.1_7C144_Restore.ipsw
3.1.0 (3GS): iPhone2,1_3.1_7C144_Restore.ipsw
3.1.2 (2G): iPhone1,1_3.1.2_7D11_Restore.ipsw
3.1.2 (3G): iPhone1,2_3.1.2_7D11_Restore.ipsw
3.1.2 (3GS): iPhone2,1_3.1.2_7D11_Restore.ipsw

NOTE*: I will update this as new firmware versions come out of course