Open source project:
http://code.google.com/p/ziphonegui/
Download:
http://ziphonegui.googlecode.com/files/ZiPhoneGUI.exe
A screenshot of ZiPhoneGUI:

ZiPhone
ZiPhoneGUI is a wrapper for Zibris ZiPhone project which is a command line utility for all iPhone unlocking tasks. For both Windows and Mac.
visit:
http://www.ziphone.org/
Download ziPhone (no GUI):
http://downloadziphone.org/ZiPhone2.4b.zip
I personally didn't test it. But it defenitely seems good.