Riva is the the best of the rest, unfortunately. I tried it a while ago after searching for hours upon hours for a decent one that doesn't mess up the audio/video sync, and it appears all of them use ffmpeg, and thus, yield the same results.
Riva will work for some flvs, but if you have one with a still picture, it will interpret it as a single frame for as long as the picture remains on screen. Thus shortening the video and messing up the sync.

All except <a href="http://www.moyea.com/">Moyea Flash 2 Video Converter</a>. It actually uses a live capture of the video rather than processing the raw data directly, so it worked perfectly. The only drawback is that it leaves a nasty watermark in the middle of the video until you pay for it. :{

Suffice to say I didn't want the video <i>that</i> badly.