I use perian on os x. It reads flv files directly in quicktime, so i can open them in gem also.

http://www.coolosxapps.net/2007/02/15/perian-05-the-swiss-army-knife-for-quicktime/

ffdshow wraps all of ffmpeg, including post processing, into a DirectShow and VideoForWindows (VfW) filter. I use it quite a bit for GEM on Windows.