Play Video (AVI, DIVX, MP4, MKV) On Android Phones Using RockPlayer
One thing you will surely miss on your Android phone is a good media player that can play most of the popular formats. RockPlayer is a free Android media player app that is still in Beta (and hence not available in the Android market yet) and can play almost any video file, including AVI, MKV, FLV, MP4, 3GP, WMV, MOV, ASF, DIVX, XVID, H.264, and RMVB.
With RockPlayer, you no longer have to worry about converting video to play on your Android. RockPlayer works with Android 1.6 to 2.2. It does not support HTC G1, G2, and G3, as it has CPU requirements of ARM V6 or ARM v7

Download RockPlayer Beta For Android
Go to RockPlayer Official Google Groups Site
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Well, I have Rockplayer installed, which plays xvid/avi fine locally, but when I want to play the same movie through DropBox I get “Sorry but there is no viewer on this phone for this filetype”. I have an HTC Desire with Android 2.2
How can you set this player as default player for all movies?