CorePlayer is a commercial evolution of the free but discontinued TCPMP. It is a full featured video and audio player that comes with its own codecs and has built in Youtube support. More info »
I used this on my Treo 700wx (WM), which basically sucks, but this player made the phone just a little more fun to use before I got my iPod touch. Plays divx/mpg fine, didn't really try other formats.
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