Notices

Is there any player that supports .mkv video format...??

Yes. The built-in Media Player supports the MKV container format if you install my Extra Decoders Support package (don't forget to donate!). However this does *not* magically add support for all the formats and HD resolutions. Refer to the troubleshooting page.

You can also try installing MPlayer and SiB (SiB is the graphical interface for MPlayer). There's also VLC, however I'm not sure about MKV support in VLC on the N900 (the desktop version definitely supports MKV, so I guess it should work).

The simplest way is to install TVersity.
Then you can have streams sent over WiFi or 3G in any format you like

Really? Who would watch streaming videos via WLAN, when they are most likely at home with bigger screens? Or using 3G data and all the issues associated with that (battery drain, bandwith cost). There's enough storage space on the N900 to copy files over and play them locally without problems. If this was any other phone I would be likely to agree, but you have the full GStreamer stack and the best multimedia playback applications here (MPlayer, VLC), there's no need for streaming unless it's really necessary (transcoding on the fly from HD resolutions, although MPlayer does handle *some* files in HD resolutions, haven't tested VLC thoroughly yet).

Thanks a lot. But I have tried that but in vain. does not work that well. Video skips and audio does not play.

What exactly have you tried? Streaming or local playback? Which application on the N900 did you use? What are the video file parameters?

Well then you're telling us right there that the video plays. So your N900 does support .mkv playback. Otherwise it wouldn't open at all.

Your video skipping is probably due to an encoded bit rate that is higher then your N900 is able to play back smoothly at the default CPU clock frequency. You could re-encode your video at a lower bit rate or try over clocking your N900 ( I know that video playback performance is mainly governed by the GPU, but it's worth a shot...)

The audio not playing is usually due to the audio in your .mkv container being encoded with some incompatible format like OGG Vorbis or something that you don't have the codec to play back on your N900. You could re-encode your video with standard mp3 audio or install OGG-Vorbis support to your N900.

EDIT: Just read this from the extras-decoders support troubleshooting page:
"None of the added formats are hardware-accelerated. This means that they need a lot of processing power from the CPU. Even though the CPU in the N900 is powerful, there are limits."

So I guess that over clocking your N900 could help quite a bit.

VLC is suppose to be hardware accelerated through the DSP and GFX chips (I can watch some shows without overclocking). It works quite well for the smaller MKV's (limited to 480p), but the problem is still with memory searches and larger files.

Hopefully the next versions will fix this.

the troubleshooting page doesnt shows any support info for .MKV format..., n whenever I try to play .rmvb file no matter what its resolution is, it totally sucks to play the file frame by frame.... I hate that...

I use knots 2 and that streams any file over including 720p MKV files, and no problem over the 3G connection.
Only thing is I don't use this app as much but its handy if your out all the time and you forget a few files or dont want to use up space on the device or if you have your cables with you then you could just hook it up to the tv and watch it in full screen whilst streaming it.
But in all honesty I would eventually like to see a codec available for the n900 that supports mkv fully just as it does with the divx etc.
And with the new tarrifs changing for 3G usage the streaming costs will soon start to hit folk in the pockets.