Last month, the iPhone hacking community had released a tool called pHaseBanDowngrader to help iPhone 3G users who had accidentally upgraded to iPhone OS 2.2.1 without preserving their baseband to downgrade baseband from 02.30.03 to 02.28.00.
However, the tool had two limitations, it worked with only bootloader version 5.08 and it was only meant for those who were technically inclined as it required some command line work.
w1kedZ (coding) & DHowett (GUI) have just released 3G Fuzzyband Downgrader to address the second issue. It is based on the same exploit in the 5.08 bootloader found by geohotz but comes with a nice and simple GUI.
It also checks the version of bootloader on your iPhone 3G and tells you if you can downgrade the baseband.