There aren’t many users that haven’t heard of the VLC media player. It’s free software and is highly portable, but is there an N900 port? There surely is.
Green screen, Maemo screenshooter can't capture video streams. But the video plays. Really.
Note: the port requires you to fetch all the dependencies, and it will chomp up around 45 mB of your memory. Since some of them aren’t well optified, you’d want at least 20 mB in your rootfs.
The VLC port doesn’t have a package in the official Maemo repos yet, but VideoLAN’s repo has all the packages you need. Go and add it, either via the Application Manager (Name: VLC; Address: http://downloads.videolan.org/videolan/maemo/; Distro: freemantle; Components: free), or via Xterm: