Flashtool can be downloaded on the Flashtool Homepage (on Installation tab)
As Flashtool was originally developped for X10, you can find a lot of information on the Original X10 Flashtool thread
Major new feature : Fastboot toolbox for 2011 line devices.
Major update for 0.6.8.0 : Ability to now install CWM from Flashtool on locked bootloader.
Fastboot now works fine under linux
Please post fastboot issues here in this thread
If ever you want to setup an eclipse environment for Flashtool, you can check this thread