Garden of Eden Creation Kit (GECK)
Download here:
http://static.bethsoft.com/fo3/geck.zip
and the update
http://download.bethsoft.com/patches/geck/GECK_1.5_Update.exe
Regarding updates (v1.5 current) and installation see:
http://geck.bethsoft.com/index.php/Bethsoft_Tutorial_GECK_Setup
The Eden Kit page for everything related to it:
http://geck.bethsoft.com/index.php/Main_Page
Updates for FO3: (currently v1.7)
http://fallout.bethsoft.com/eng/downloads/updates.html