The baseband bootloader is the code which runs before the baseband FW, it is responsible for signature checking and updating the baseband. See also bootloader.
S-Gold 2 Revisions
Found in the iOS 1.0 and 1.1.1 Ramdisks for the iPhone. It was often (unintentionally) installed when someone with a Fakeblanked bootloader 3.9 downgraded to 1.0 or 1.1.1. This can be re-updated with BootNeuter.
X-Gold 608 Revisions
This is the bootloader from the iPhone 3G/X-Gold 608. It is, in contrast to 3.9 and 4.6, sig checked on startup. There is an exploit where the main fw cert is passed with the loader instead of the loader cert, and it checks the main firmware instead, allowing you to upload unsigned loader code. This has been fixed in 5.9. You can downgrade from 02.30.03 to 02.28.00 using pHaseBanDowngrader (by pH) in the Bootloader 5.8 or from 05.14.02,05.15.04 and 06.15.00 in the 5.8 Bootloader with Fuzzyband Downgrader. It once also downgraded 05.11.07, 05.12.01 and 05.13.04 to 04.26.08.
This is the bootloader of version 2.1 and 2.2 OTB (and some 2.0 OTB) iPhone 3G/X-Gold 608. Still has no known exploits and it was released as soon as Apple knew iPhone Dev Team could downgrade their iPhone 3G baseband from 1.48 to 1.45. Now, all the iPhone 3G has bootloader 5.9 and higher.
This is the latest bootloader of version 2.2.1 OTB in 2008 iPhone 3G/X-Gold 608. Still has no known exploits and it was released as soon as Apple knew iPhone Dev Team could unlock their iPhone 3G baseband version 2.28 by yellowsn0w. Now, all the iPhone 3G 2.2.1 OTB has bootloader 6.2.
This is the latest bootloader of version 2.2.1 OTB in 2009 iPhone 3G/X-Gold 608. Still has no known exploits and it was released as soon as Apple released firmware 3.x beta for testing. iPhone 3GS and iPad 3G+WiFi units contain this bootloader as well. As of the 3.x firmwares (baseband v. 4.x), the baseband now contains the loader of bootloader 6.4.
X-Gold 618, MDM6600, MDM6610 Revisions
This is the bootloader that ships with the iPhone 4. It has no known exploits. It is in a zip file with the baseband in the iPhone firmware. The name of the zip file in iOS 4.0-4.0.2 is ICE3_01.59.00_BOOT_02.06.Release.bbfw.
This is a new bootloader for the iPhone 4, the bootloader was updated at 4.1 and used on iPhone 4 4.1-4.2.1 OTB. It doesn't allow a downgrade, even if the older baseband is still signed from Apple. It has no known exploits. It is in a zip file with the baseband in the iPhone firmware. The name of the zip file in iOS 4.1 is ICE3_02.10.04_BOOT_02.08.Release.bbfw.
This is a new bootloader for the iPhone 4 and iPad 2 (iPad2,2) and ships with iOS 4.3-5.0 OTB. The name of the baseband file in iOS 4.3.3 is ICE3_04.10.01_BOOT_02.13.Release.bbfw and doesn't allow baseband downgrades also if the baseband is still signed by Apple. Also this version is being used in the initial shipment of the iPhone 4S.