Flashtool can be downloaded on the Flashtool Homepage
As Flashtool was originally developped for X10, you can find a lot of information on the Original X10 Flashtool thread
Major update for 0.6.8.0 : Ability to now install CWM from Flashtool on locked bootloader for LT15/LT18.
Fastboot now works fine under linux
Major feature for 0.6.5.0 is Linux and Windows editions, Drag&Drop for FTF files.
Major new feature : Fastboot toolbox for 2011 line devices.
Please post fastboot issues here in this thread