CorePlayer (formerly BetaPlayer) is the best video player I've ever seen. I used to have it on my iPaq and it was smooooth.
It's being released for Synbian 60 v3 on the 29th of August (probably US time). [edit] now released.
See HERE for betaboy's post on the CorePlayer blog.
See HERE to see it working on Youtube on the N80.
Betaplayer used to be free, but I think they'll charge for this
It will be worth it though! Much better than the RealPlayer that comes installed on the N95.
CorePlayer front page HERE
Here's an update by Betaboy no the file types it can play -
A1:AUDIO: MP3, MP2, AAC, H.263, WAV, OGG, Speex, WAVPACK, FLAC, MPC, TTA, AMR, ADPCM, ALaw, MuLaw, Midi
A2:Video: H.264 (CoreAVC), MPEG-1, MPEG-4 part 2 (ASP), DivX, XviD, Theora, Dirac, VP3, MJPEG, NRE, MSVIDEO1, and FLV*
*The FLV container for Flash video is supported in CorePlayer Mobile however not available in our consumer version and only available to OEM's if they have a Flash license directly from Adobe.