What does Netscape have to do with Cygwin, you ask? Didn't I just say that anything Mozilla-related doesn't work yet on Cygwin?
Fortunately we still have one working browser on Cygwin: Konqueror. The nsplugins package allows Konqueror to load and use Netscape-compatible plugins. At the next upload, there will be two such working plugins: DjView3 and Xine. I tried, but failed, to get swfdec-mozilla 0.5 working as well; another try with 0.6 will have to wait for GNOME 2.22.
Yes, swfdec itself and the swfdec-gnome player are working, but I had to disable sound support. Enabling the OSS sound support would lock up video playback while the audio was playing. In the meantime, at least it works for games.
If you're looking to play YouTube videos within Cygwin, a recent Linux.com article lists several downloaders. FFmpeg support allows the following applications to play the FLV videos: ffplay (ffmpeg), gxine, kaffeine, kmplayer, kplayer, mplayer, totem, vlc, xine (xine-ui).