Here's a list of some well-known formats that YouTube supports:
WebM files - Vp8 video codec and Vorbis Audio codecs
.MPEG4, 3GPP and MOV files - Typically supporting h264, mpeg4 video codecs, and AAC audio codec
.AVI - Many cameras output this format - typically the video codec is MJPEG and audio is PCM
.MPEGPS - Typically supporting MPEG2 video codec and MP2 audio
.WMV
.FLV - Adobe-FLV1 video codec, MP3 audio
*from youtube