Most videos i find on the internet dont work on my N770.
Even files of a "supported type"s (i.e. such files, which offer an "open" in the download-dialog (i.e. mpg, mpeg, ..)) mostly are not played by the video player.

Which filetypes should work, an what might be the reason when such files dont?

Many "video sites" seem to require flash (7 or 8 or something other not available on 770). Is there any possibility to watch those videos without flash?