The Windows Phone app which has gone free next as myAppFree app of the day is FLV Player.
FLV Player Details:
FLV Player is ultra fast, tiny video player for FLV, MKV, VOB, MPG, MP4, TS and many others video files. Player has embedded video and audio codecs that needed to play most video files. Supports embedded and external subtitles. Hardware-optimized video and audio decoding saves battery and allows you to play more movies.